of 11
8/11/2019 Cod , n Balance in Stp Model
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ergamon
0043- 1354( 94) 00155- 3
Wat Res
Vol . 29, No. 2 , pp. 633-643, 1995
C op yr i gh t 1995 E l s e v i e r S c i e nc e L t d
P r i n t e d i n G r e a t B r i t a i n . A l l r i gh t s re s e r ve d
0043-1354/95 7.00 + 0.00
C O D N D N I T R O G E N M S S B L N C E S I N C T I V T E D
S L U D G E S Y S T E M S
P. S . BARKER an d P. L. DOLD*
Dep artm ent of Civil Engineering, M cM aster University, 1280 M ain Street West, Hamilton, O ntario,
Cana da L8S 4L7
First received December 1993; accepted in revised orm Ma y 1994)
A ~t ra ct- -C OD and ni trogen balances we re per formed on four dif ferent types of laboratory-scale
activated sludge system: aerobic, anoxic, anox ic-aero bic and an aero bic-a no xic- aero bic biological excess
pho spho rus removal systems). The systems included a variety o f configurations, with differing wastewater
characteristics and ope rating parameters. The results suggest that goo d COD balances are to be expected
in aerob ic and anox ic-aerobic systems. Systems incorporat ing anaerobic zones exhibi t low C OD balances
( less than 80%). Ferme ntat ion in the anaerobic zone apparently is implicated in this loss of COD. The
consequences of the COD loss include bot h a significant decrease in oxygen requirements an d in sludge
production comp ared to aerobic or an oxic-aerob ic systems. Possible mechanisms for the loss of COD and
areas which require further study are discussed.
Key words--activated sludge, mass balances, CO D, T KN , nitr if ication, denitr if ication, biological excess
pho spho rus removal (BEPR), fermentation, anae robic stabilization
I N T R O D U C T I O N
C u r r e n t u n d e r s t a n d i n g o f a c t i v a t e d s l u d g e s y s t e m
b e h a v i o u r h a s d e v e l o p e d f ro m o b s e r v a t i o n s o n o p e r -
a t i n g s y s t e m s . T h e s e h a v e b e e n f u l l - s c a l e p l a n t s , p i l o t
p l a n t s a n d l a b o r a t o r y - s c a l e e x p e r i m e n t a l s y st e m s .
T h e r e i s a f u n d a m e n t a l r e q u i r e m e n t i f c o n c l u s i o n s
d r a w n f r o m a n a l y s i s o f e x p e r i m e n t a l d a t a a r e d e fe n -
s i b l e ; n a m e l y , t h a t d a t a g a t h e r e d f r o m t h e e x p e r -
i m e n t a l s y s t e m a r e r e l i a b l e . M a s s b a l a n c e s p r o v i d e
o n e w a y o f c h e c k i n g t h e r e l ia b i l i ty o f t h e d a t a .
S u r p r i s i n g l y t h i s i s s e l d o m d o n e , m o s t l i k e l y b e c a u s e
g a t h e r i n g t h e d a t a t o c o n d u c t t h e s e b a l a n c e s m a y
n e c e s si t a te a d d i t i o n a l s a m p l i n g a n d m o n i t o r i n g o f t h e
e x p e r i m e n t a l s y s t e m , b e y o n d t h a t r e g a r d e d a s n e ce s s-
a r y f o r a d d r e s s i n g a p a r t i c u l a r r e s e a r c h p r o b l e m .
A l s o , i n c e r t a i n c a s e s i t m a y n o t b e f e a s i b l e t o g a t h e r
t h e r e q u i r e d d a t a ; f o r e x a m p l e , o n a f u l l - s c a le p l a n t
w i t h d y n a m i c i n f l u e n t l o a d i n g .
T w o b a l a n c e s w h i c h o f t e n c a n b e a p p l i e d t o e x p e r -
i m e n t a l d a t a a r e f o r c h e m i c a l o x y g e n d e m a n d ( C O D )
a n d n i t r o g e n ( N ) . I n a d d i t i o n t o r e f le c t i n g t h e v a l i d i t y
o f e x p e r i m e n t a l d a t a , r e s u lt s o f m a s s b a l a n c e c a l c u -
l a t i o n s c a n a l s o b e u s e d t o i n v e s t i g a t e p r o c e s s b e -
h a v i o u r , l e a d i n g t o a n i m p r o v e d u n d e r s t a n d i n g o f t h e
u n d e r l y i n g m e c h a n i s m s .
T h i s p a p e r r e v ie w s th e r e s u l ts o f C O D a n d n i t r o g e n
m a s s b a l a n c e s o n l a b o r a t o r y - s c a l e a c ti v a t e d s l u d g e
s y s te m s w i t h a r a n g e o f c o n f ig u r a t i o n s , o p e r a t i n g
p a r a m e t e r s a n d i n f l u e n t w a s t e w a t e r c h a r a c t e r i s t i c s .
*Author to whom all correspondence should be addressed.
I n t h e d i s c u s s i o n o f r e s u l t s , e m p h a s i s i s o n n u t r i e n t
r e m o v a l s y s t e m s . I n t h e b i o l o g i c a l e x c e s s p h o s -
p h o r u s r e m o v a l ( B E P R ) s y s te m s i n v e s t ig a t e d in
t h i s s tu d y , t h e r e s u l t s o f C O D b a l a n c e s s u g g e s t t h e
p o s s i b i l i t y o f f e r m e n t a t i v e p r o c e s s e s o c c u r r i n g i n t h e
a n a e r o b i c z o n e , l e a d i n g t o a l o s s o f C O D f r o m
t h e s y s t e m .
D A T A B A S E F O R C O D N D N I T R O G E N M A S S
B A L A N C E S
C o m p r e h e n s i v e d a t a a r e re q u i r e d to p e r f o r m C O D
a n d N m a s s b a l a n c e s . T h e d a t a s e t i n c l u d e s i n f o r -
m a t i o n o n o p e r a t i n g p a r a m e t e r s ( f l o w s , r e c y c l e r a t e s ,
s l u d g e w a s t a g e , e t c . ) , i n f l u e n t a n d e f f l u e n t c o n c e n -
t r a t i o n s o f C O D , T K N a n d n i t r a te , a s w el l a s t h e
c o n c e n t r a t i o n o f n i t r a t e i n e a ch r e a c t o r, t h e o x y g e n
u t i l i z a t i o n r a t e (s ) a n d t h e v o l a t i l e s u s p e n d e d s o l i d s
( V S S ) c o n c e n t r a t i o n o f t h e w a s t e s l u d g e . In a d d i t i o n ,
t h e C O D / V S S r a t i o a n d t h e T K N / V S S r a t i o s h o u l d
a l s o b e r e p o r t e d .
T h e c o m p l e t e d a t a s e t s e l d o m i s a v a i l a b l e f o r
s y s t e m s w h i c h r e c e i v e a t i m e - v a r y i n g ( i. e . d y n a m i c
i n p u t o f fl o w a n d / o r c o n c e n t r a t i o n . A l s o , f o r d y -
n a m i c s y s t e m s , i t i s n e c e s s a r y t o c o n s i d e r a c c u m u -
l a t i o n t e r m s i n m a s s b a l a n c e c a l c u l a t i o n s ; t h i s
c o m p l i c a t e s t h e p r o c e d u r e c o n s i d e r a b l y . A l l o f t h e
s y s te m s c o n s i d e r e d i n t h i s p a p e r w e r e o p e r a t e d u n d e r
s t e a d y s t a t e c o n d i t i o n s a s fa r a s is p r a c t i ca l . T h a t
i s, t h e s y s t e m s e a c h r e c e iv e d a f i x e d i n f l u e n t v o l u m e
p e r d a y a t a c o n s t a n t r a t e , a n d t h e c o m p o s i t i o n o f
t h e i n f l u e n t w a s n e a r l y u n i f o r m f r o m d a y t o d a y .
A l s o , a f i x e d v o l u m e o f m i x e d l i q u o r w a s r e m o v e d
633
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634 P.S. BARKERand P. L. DOLD
from the systems each day to maintain a constant
sludge age (SRT, MCRT). Where municipal waste-
water was used as inf luent, the influent was prepared
by diluting high-strength wastewater to a target
COD concentration for the duration of each study.
Because the wastewater was drawn from the
same source, the composition (TKN/COD ratio,
unbiodegradable fractions, etc.) remained reasonably
constant.
An impor tant aspect regarding steady-state data is
that the system necessarily should be operated for an
extended period (3-4 sludge ages) to ensure attain-
ing a steady operating condition. Even under this
condition there will be fluctuations in the values of
monitored parameters from day to day. Therefore
it is essential that data used in mass balances
should be averages obtained over an extended period
after attaining steady state. This will account for
small fluctuations in response. Also, averaging over
an extended period avoids the necessity to include
accumulation terms in mass balance calculations. In
the mass balances reported here the steady-state data
were gathered in this manner.
Data from five sources for four different types of
activated sludge system were used in this investi-
gation:
aerobic systems--Schroeter
e t a l
(1982)
anoxic only systems- -McClint ock e t a l (1988)
anoxic-aerobic systems--Arkley and Marais
(1981)
anaerobic-anoxic-aerobic systems--Wentzel
e t a L
(1989, 1990).
C O D MASS BALANCES ON EXPERIMENTAL DATA
The premise of a COD balance is that it should be
possible to account for the COD entering an acti-
vated sludge system via the influent in the following
fractions:
COD (unfiltered) leaving the system in the
effluent
o COD incorporat ed into the sludge mass
through cell synthesis, enmeshment or
absorption, leaving the system in the sludge
wastage stream
COD oxidized (i.e. the elec trons which are
transferred from the organic material to the
electron acceptor). In purely aerobic systems
this fraction can be estimated from the oxygen
utilization rate (after deducting the oxygen
required for nitrifi cation). In systems incorpo-
rating anoxic zones (that is, where nitrate is
present but oxygen is not), it is also necessary
to account for COD oxidized through deni-
trification.
The assumption here is that any COD loss due
to volatilization of organics is negligible. Also, it
is assumed that denitrification under aerobic con-
ditions is not significant. These aspects are discussed
later.
In systems incorporating denitrification, the
mass of COD oxidized through denitrificat ion can be
accounted for by estimating the equivalent amount of
oxygen which would have been needed if oxygen had
been the electron acceptor instead of nitrate. It has
been proposed that denitrification is essentially a four
step process (Payne, 1981).
NO3 --, NO{ ~ NO ~ N2 0 ~ N2
Each step may be represented by a half reaction
where e- denotes electron equivalents (COD) trans-
ferred from the organic substrate:
2e- + NO 3 + 2H ~ NO~- + H2 0
e- + N O / + 2H --* NO + H20
2e + 2NO + 2H ---, N2 0 + H2 0
2e- + N20 + 2H ---, N2 + H20
The net reaction is obtained by combining the four
equations:
10e- + 2NO3 + 12H ~ N2 + 6H20
or equivalently,
e +-~NO3 6 + 1 3
- +~H --* i-6N2 + ~H20
Similarly, the half reaction for the reduction of
oxygen is given by:
1 H 1
e- +~O 2+ --* iH2 0
The above two equations imply that the transfer of
one electron equivalent requires the reduction of
1/4 mol o f oxygen or 1/5 mol of nit rate , i.e.:
-~ mol nitr ate - ~ mol oxygen
g NO 3 - N - 32
~- g oxygen
1 mg NOa -- N - 2.86 mg oxygen
The assumption here is that nitrate denitrified is
converted to nitrogen gas (N2), and that there is no
release of intermediates (NO2, NO, N20). If inter-
mediates were released the factor of 2.86 would be
lower.
A detailed description of the COD mass balance
procedure is described in the Appendix.
NITROGEN MASS BALANCES ON EXPERIMENTAL
DATA
In general, nitrogen enters the system in the
form of organic N or ammonia. The influent TKN
gives a measure of the amount of these compounds
present. If the system is nitrifying, the majority of
the influent TK N is converted to nit rate. I f the
system includes unaerated zones, then denitrifi-
cation will result in the conversion of a portion of
the nitrate to nitrogen gas (or the intermediates
discussed above). Nitrogen leaving the system in
the gaseous form can be estimated by performing a
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CO D and nitrogen mass balances 635
T
Fig. 1. Aero bic system configuration utilized by Schroeter e t
a l (1982).
NO
Fig. 2. Anoxic system configuration utilized by McC lintock
e t aL (1988).
n i t r a t e b a l a n c e o n t h e u n a e r a t e d r e a c t o r s . A f r a c t i o n
o f t h e i n f l u e n t T K N i s a l s o u t i l i z e d i n c e ll s y n th e s i s ,
l e a v i n g t h e s y s t e m i n t h e s l u d g e w a s t a g e s y s t e m .
T h e r e f o r e , i t s h o u l d b e p o s s i b l e t o a c c o u n t f o r t h e
m a s s o f n i t r o g e n i n t h e i n f l u e n t in t h e f o l l o w i n g
f r a c t i o n s
e f f l u e n t T K N ( u n f i l t e r e d )
e f f luen t n i t r a te
T K N o f t h e w a st e s lu d g e
n i t r o g e n c o n v e r t e d t o g a s e o u s n i t r o g e n
t h r o u g h d e n i t r i fi c a t i o n (i f th e s y s t em i n c lu d e s
u n a e r a t e d z o n e s ) .
A s w i t h t h e C O D b a l a n c e , a e r o b i c d e n i t r if i c a ti o n is
n o t c o n s i d e r e d h e r e. A s i g n if i ca n t a m o u n t o f a e r o b i c
d e n i t r i fi c a t i o n i n a s y s te m w o u l d r e s u lt i n l o w C O D
a n d N b a l a n c e s .
A d e t a i l e d d e s c r i p t i o n o f t h e n i t r o g e n m a s s b a l a n c e
p r o c e d u r e i s d e s c r i b e d i n t h e A p p e n d i x .
AEROBIC SYSTEMS
S c h r o e t e r e t a l ( 1 9 8 2 ) o p e r a t e d f o u r p a i r s o f
a e r o b i c l a b - s c a l e sy s t e m s a t t e m p e r a t u r e s o f 1 2 a n d
2 0 C . T h e s y s t e m s w e r e o p e r a t e d a t s l u d g e a g e s o f
3 , 8 a n d 2 0 d a y s ; t w o s y s t e m s w e r e o p e r a t e d a t t h e
3 d a y s l u d g e a g e f o r r e a s o n s d i s c u s s e d l a t e r . T h e
i n f l u e n t c o n s i s t e d o f d o m e s t i c w a s t e w a t e r , d i l u t e d
w i t h t a p w a t e r t o a c o n c e n t r a t i o n o f 5 00 m g C O D / I .
A l l s y s t e m s w e r e s i n g l e r e a c t o r s y s t e m s a s s h o w n
in Fig. 1.
T h e r e s u l ts o f m a s s b a l a n c e s f o r C O D a n d N
f o r t h e S c h r o e t e r
e t a l
( 1982) sys tems a r e l i s ted in
T a b l e I . T h e r e s u l t s a r e v e r y g o o d f o r a l l s y s t e m s ,
r e g a r d l e s s o f w h e t h e r n i t r i f i c a t i o n w a s o c c u r r i n g
( s y s t e m s a t t h e 3 d a y s l u d g e a g e a n d 1 2 '~ C w e r e n o t
n i t r i f y i n g ; a l l o t h e r s y s t e m s n i t r i f i e d f u l l y ) . A s t h e
n i t r o g e n b a l a n c e s a r e c l o s e t o 1 0 0 , i t i s u n l i k e l y t h a t
a e r o b i c d e n i t r i f i c a t i o n o c c u r r e d i n t h e s e s y s t e m s t o a
s i g n i f i c a n t e x t e n t .
I n i t ia l l y o n l y o n e s y s t e m w a s o p e r a t e d a t a 3 d a y
s l u d g e a g e ; h o w e v e r , th e C O D b a l a n c e o b t a i n e d a t
1 2 C w a s p o o r ( 8 1 . 7 v e r s u s N b a l a n c e o f 9 7 . 4 ) .
E x a m i n a t i o n o f t h e s y st e m i n d i c a te d t h a t i n a d v e r t e n t
a e r a t i o n d u e t o t u r b u l e n c e a t t h e r e a c t o r s u r f a c e w a s
t h e c a u s e , l e a d i n g t o u n d e r e s t i m a t e s o f t h e o x y g e n
u t i l iz a t i o n r a t e . W h e n t h e p r o b l e m w a s r e m e d i e d
( b y u s i n g a s m a l l e r s t i r r e r p a d d l e ) t w o s y s t e m s w e r e
o p e r a t e d i n p a r a ll e l a n d v e r y g o o d C O D a n d N
b a l a n c e s w e r e o b t a i n e d f o r b o t h . T h i s h i g h l i g h t s t h e
n e e d f o r c a r e f u l a t t e n t i o n t o e x p e r i m e n t a l d e t a i l , a n d
t h e u t i l i t y o f m a s s b a l a n c e s i n c h e c k i n g t h e v a l i d i t y o f
e x p e r i m e n t a l d a t a .
ANOXIC ONLY SYSTEMS
M c C l i n t o c k e t a l ( 1 9 88 ) o p e r a t e d l a b - s c a l e s in g l e
r e a c t o r a e r o b i c a n d a n o x i c s y s t e m s i n p a r a l l e l a t f i v e
d i f f e r e n t s l u d g e a g e s . T h e 3 d a y s l u d g e a g e s y s t e m s
w e r e o p e r a t e d a s f l o w t h r o u g h s y s t e m s ; s y s t e m s a t
l o n g e r s l u d g e a g e s w e r e o p e r a t e d a s s i n g l e r e a c t o r
s y s t e m s w i t h a b a f f l e i n s e r t e d t o s e p a r a t e t h e m i x e d
l i q u o r z o n e f r o m t h e s l u d g e s e tt l i n g z o n e . F u n c t i o n -
a l l y t h e s y s t e m s a r e s h o w n s c h e m a t i c a l l y i n F i g . 2 .
A l l s y s t e m s r e c e i v e d a s y n t h e t i c s e w a g e ( p r i m a r i l y
b a c t o p e p t o n e ) , w i t h n i t r a t e a n d o x y g e n s u p p l i e d i n
e x c e s s t o t h e a n o x i c a n d a e r o b i c r e a c t o r s , r e s p e c t -
i v e ly . U n f o r t u n a t e l y , a s o x y g e n u t i l i z a t i o n r a t e s w e r e
n o t r e p o r t e d , C O D b a l a n c e s c a n n o t b e c o n d u c t e d o n
t h e a e r o b i c s y s t e m s .
R e s u l ts o f m a s s b a l a n c e s fo r C O D a n d N f o r
t h e M c C l i n t o c k e t a l ( 1988) sys tems a r e l i s ted
i n T a b l e 2 . C O D b a l a n c e s o n t h e a n o x i c s y s t e m s
r a n g e d f r o m a p p r o x . 95 to 8 5 , w i t h a d e c r e a s i n g
t r e n d w i t h i n c r e a s i n g s l u d g e a g e ( e x c e p t f o r t h e
1 5 d a y s y s t e m ) ; n i t r o g e n b a l a n c e s a r e r e a s o n a b l e
f o r a l l s y s t e m s . ( I t s h o u l d b e n o t e d t h a t t h e d a t a
f r o m t h e 1 5 d a y a n o x i c s y s t e m i s q u e s t i o n a b l e a s
s o l i d s p r o d u c t i o n i n t h i s s y s t e m w a s r e p o r t e d l y
g r e a t e r t h a n t h a t f o r t h e 1 5 d a y a e r o b i c s y s t e m , w h i le
Table 1. COD and nitrogen mass balances for aerobic systems [data from Schroeter
et a
(1982)]
N balance ( ) COD balance ( )
Sludge age
System (d) 12 C 20 C 12 C 20 C
1 3 100.2 100.5 99.6 100.4
2 3 100.2 99.5 99.7 100.2
3 8 100.2 97.5 99.6 99.9
4 20 99.2 99.4 99.4 98.4
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636
P. S. BARKERand P. L. DOLD
Table 2. COD and nitrogen mass balances for anox ic and aerobic systems
[data from M cClintock t aL (1988)]
S l u d g e a g e N b a l a n c e C O D b a l a n c e
S y s t e m T y p e ( d ) ( ) ( )
1 A e r o b i c 1 .5 1 0 7 . 9 N o t a v a i l a b l e
2 A e r o b i c 3 . 0 9 9 . 9 N o t a v a i l a b l e
3 A e r o b i c 6 . 0 1 0 3 . 3 N o t a v a i l a b l e
4 A e r o b i c I 0 . 0 9 6 . 0 N o t a v a i l a b l e
5 A e r o b i c 1 5. 2 9 8 . 8 N o t a v a i l a b l e
6 A n o x i c 1 .5 9 5 . 5 9 4 . 7
7 A n o x i c 3 . 0 9 8 . 3 9 4 . 4
8 A n o x i c 6 .1 9 5 . 4 9 1 . 0
9 A n o x i c 9 . 6 9 5 . 9 8 5 . 5
1 0 A n o x i c 1 5 .1 9 8 . 0 9 3 . 6
t h e o t h e r a n o x i c s y s t e m s w e r e o b s e r v e d t o p r o d u c e
a p p r o x . 4 0 l es s s o l id s t h a n t h e c o r r e s p o n d i n g
a e r o b i c s y s t e m s ) . C o m p a r i n g t h e s e C O D b a l a n c e
r e s u l t s w i t h t h o s e f o r t h e S c h r o e t e r e t a l ( t 9 8 2 )
a e r o b i c s y s t e m s , i t a p p e a r s t h a t c o m p l e t e l y a n o x i c
s y s t e m s ( e s p e c i a l l y a t l o n g e r s l u d g e a g e s ) m a y e x h i b i t
p o o r e r b a l a n c e s .
I n c o n s i d e r i n g p o s s i b l e e x p l a n a t i o n s f o r t h e l o w e r
C O D b a l a n c e s i n a n o x i c s y s t e m s , i t i s u s e f u l t o
c o n s i d e r t h e a s s u m p t i o n s m a d e i n p e r f o r m i n g a
C O D b a l a n c e o n a n a n o x i c s y s t e m . T h e i m p l i c i t
a s s u m p t i o n w i t h a n o x y g e n e q u i v a l e n c e f a c to r o f 2 . 86
f o r n i t r a t e d e n i t r i f i e d i s t h a t d e n i t r i f i c a t i o n i s c o m -
p l e t e . I f d e n i t r i f i c a t i o n i n t e r m e d i a t e s s u c h a s n i t r i c
o x i d e ( N O ) o r n i t r o u s o x i d e ( N : O ) w e r e r e le a s e d t o
t h e a t m o s p h e r e , f e w e r e l e c tr o n s w o u l d b e t r a n s f e r r e d
p e r u n i t n i t r a t e d e n i t r i f i e d , a n d t h e e q u i v a l e n c e f a c t o r
w o u l d b e l o w e r t h a n 2 . 86 . I f th i s w e r e t h e c a s e , th e
a c t u a l C O D b a l a n c e w o u l d b e ev e n lo w e r t h a n t h a t
c a l c u l a t e d u s i n g a f a c t o r o f 2 .8 6 . A s t h e s e a n o x i c
s y s t e m s t e n d t o h a v e l o w C O D b a l a n c e s , i t s e e m s
u n l i k e l y t h a t t h e r e l e a s e o f d e n i t r i f i c a t i o n i n t e r m e d i -
a t e s is a m a j o r f a c t o r ; t h e r e f o r e o t h e r p o s s i b l e c a u s e s
f o r t h e l o w e r C O D b a l a n c e s s h o u l d b e c o n s i d e r e d
( t h e s e a r e d i s c u s s e d l a t e r ) .
A N O X I C A E R O B I C S Y S T E M S
A r k l e y a n d M a r a i s ( 19 8 1 ) o p e r a t e d l a b - s c a le
a e r o b i c , p r e - d e n i t r i f ic a t i o n a n d p o s t - d e n i t r i fi c a t i o n
s y s t e m s . A l l s y s t e m s w e r e o p e r a t e d a t a 2 0 d a y
s l u d g e a g e w i t h m u n i c i p a l w a s t e w a t e r a s i n f l u e n t .
S y s t e m 1 ( o p e r a t e d a s a c o n t r o l ) w a s a s i n g l e r e a c t o r
a e r o b i c s y s t e m ( F i g . 1 ) . S y s t e m s 2 , 3 a n d 4 w e r e
t w o - i n s e r i e s r e a c t o r c o n f i g u r a t i o n s o p e r a t e d i n
t h r e e p o s s i b l e m o d e s : ( i ) b o t h r e a c t o r s a e r a t e d ; o r
( ii ) f i rs t r e a c t o r u n a e r a t e d ( p r e - d e n i t r i f i c a t io n ) ; o r
( i i i ) s e c o n d r e a c t o r u n a e r a t e d ( p o s t - d e n i t r i f i c a t i o n ) .
T h e p r e - a n d p o s t - d e n i t r i f i c a t i o n s y s t e m c o n f i g u r -
a t i o n s a r e s h o w n i n F i g . 3 . D a t a f o r t h e e x p e r i m e n t a l
s t u d y w e r e r e p o r t e d f o r a n u m b e r o f p h a s e s c o r re -
s p o n d i n g t o d i f f er e n t o p e r a t i n g m o d e s . F o r e x a m p l e ,
S y s t e m 2 w a s o p e r a t e d s e q u e n t i a l l y i n a p o s t -
d e n i t r i f i c a t i o n , a p r e - d e n i t r i f i c a t i o n a n d a f u l l y
a e r o b i c m o d e i n p h a s e s I , I I a n d I I I , r e s p e c t i v e l y
( se e T a b l e 3 ) . T a b l e 3 a l s o l i st s t h e u n a e r a t e d v o l u m e
( o r m a s s ) f r a c t i o n s f o r p h a s e s w i t h u n a e r a t e d
r e a c t o r s .
A n i m p o r t a n t f e a t u re o f t h e d a t a i s th e n i t r a t e
c o n c e n t r a t i o n i n t h e u n a e r a t e d r e a c t o r i n s y s t e m s
o p e r a t e d i n t h e p r e - d e n i t ri f i c a ti o n m o d e . F o r S y s t e m
2 ( p h a s e I I , w i t h a n u n a e r a t e d v o l u m e f r a c t i o n o f 0 .4 )
t h e n i t r a t e c o n c e n t r a t i o n i n t h e u n a e r a t e d r e a c t o r w a s
a p p r o x . 1 4 m g N / 1 ; t h a t i s , t h e r e a c t o r w a s a n o x i c . F o r
S y s t e m 4 ( p h a s e s I I a n d I V , w i th a l a r g e r u n a e r a t e d
v o l u m e f r a c ti o n o f 0 .7 ) t h e n i t r a t e c o n c e n t r a t i o n
i n t h e u n a e r a t e d r e a c t o r w a s e s s e n t i a l l y z e r o ; t h a t i s ,
t h e r e a c t o r w a s a n a e r o b i c .
R e s u l t s o f m a s s b a l a n c e s fo r C O D a n d N f o r
t h e A r k l e y a n d M a r a i s ( 1 98 1 ) s y s t e m s a r e l i s t e d i n
T a b l e 3 . B a l a n c e s w e r e p e r f o r m e d o n l y f o r t h o s e
p h a s e s w h e r e t h e s y s t e m s a p p e a r e d t o b e a t s t e a d y
s t a t e . T h e f o l l o w i n g f e a t u r e s a r e e v i d e n t :
C O D b a l a n c e s o n c o m p l e t e l y a e r o b i c s y s te m s
a v e r a g e a p p r o x . 9 5
t h e p r e - d e n i t r i f i c a t i o n s y s t e m w i t h a n a n o x i c
r e a c t o r ( a s o p p o s e d t o a n a e r o b i c ) e x h i b i t s a
C O D b a l a n c e c l o s e t o 1 0 0 ( 9 6 . 9 - - S y s t e m
2, phase I I )
t h e p o s t - d e n i t r i f i c a t i o n s y s t e m w i t h a n a n o x i c
r e a c t o r (a s o p p o s e d t o a n a e r o b i c ) a l s o e x h i b i t s
a C O D b a l a n c e c l o s e t o 1 0 0 ( 97 .7 0 /0_
Sys tem 2 , phase I )
PRE DENITRIFICATION SYSTEM
POST DENITRIFICATION SYSTEM
Fig. 3. Pre- and post-denitrification system configurations
utilized by Arkley and Martin (1981).
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COD and n i t rogen mass ba lances
Table 3. C OD and nitrogen mass balances for aerobic and anoxic-aerobic systems [data from A rkley
and M arais (1981)]
Unaerated N balance CO D balance
System Phase Type fraction ( ) ( )
1 I Aerob ic 0 99.0 92.7
I 11 Aer obic 0 93.4 95.8
I I 11 Ae robic 0 99.4 94.2
2 1 Post-d enitrifica tion 0.40 99.2 97.7
2 II Pred enitrifica tion 0.40 110.4 96.9
2 III Aer obic 0 102.3 96.5
3 I11 Ae robic 0 100.4 95.3
4 II Pre den it.-- ana ero bic 0.70 105.5 77.3
4 Ill Aer obic 0 102.1 98.1
4 1V Pre den it.-- ana ero bic 0.70 93.5 74.2
637
t h e p r e - d e n i t r i f i c a t i o n s y s t e m s w h i c h m a i n -
t a i n e d e s s e n t i a l l y a n a e r o b i c c o n d i t i o n s i n t h e
u n a e r a t e d r e a c t o r b o t h e x h ib i t C O D b a l an c e s
b e l o w 8 0 ( S y s t e m 4 , p h a s e I I a n d IV )
t h e o p e r a t i n g m o d e f o r S y s t e m 4 se q u e n t i a l l y
w a s c h a n g e d f r o m p r e - d e n i t r i f i c a t i o n t o f u l l y
a e r o b i c a n d t h e n r e t u r n e d t o p r e - d e n i tr i f i ca -
t i o n i n p h a s e s I I , I I I a n d I V , r e s p e c t i v e l y . T h e
n i t r a t e c o n c e n t r a t i o n i n t h e u n a e r a t e d r e a c t o r
f o r p h a s e s I I a n d I V a c h i e v e d a s t e a d y - s t a te
v a l u e o f z e ro . T h e C O D b a l a n c e c h an g e d f r o m
7 7 .3 t o 9 8 .1 t o 7 4 . 2 a s t h e o p e r a t i n g m o d e
c h a n g e d
n i t r o g e n b a l a n c e s f o r a l l p h a s e s a v e r a g e c lo s e
t o 1 0 0 .
T h e s e o b s e r v a t i o n s s u g g e s t t h a t C O D b a l a n c e s
f o r b o t h a e r o b i c a n d a n o x i c - a e r o b i c s y s t e m s s h o u l d
b e c l o s e t o 1 0 0 . H o w e v e r , w h e n a n a n a e r o b i c
c o n d i t i o n e x i s t s i n t h e s y s t e m , t h e c a l c u l a t e d C O D
b a l a n c e i s si g n i f ic a n t l y l o w e r : th i s p h e n o m e n o n i s
d i s c u s s e d l a t e r .
A N A E R O B I C A N O X I C A E R O B I C S Y S T E M S
W e n t z e l e t a l ( 1 99 0 ) p r o v i d e c o m p r e h e n s i v e
d a t a f o r 3 0 l a b - s c a l e n u t r i e n t r e m o v a l s y s t e m s t r e a t -
i n g m u n i c i p a l w a s t e w a t e r . T h e s e s y s t e m s v a r i e d i n
c o n f i g u r a t i o n , r e a c t o r s i z e s , r e c y c l e r a t i o s , i n f l u e n t
f l o w r a t e s a n d w e r e o p e r a t e d o v e r a r a n g e o f s lu d g e
a g e s f r o m 3 to 2 1 d a y s . T h e r e w e r e f i v e b a s i c c o n -
f i g u r a t i o n s a s s h o w n i n F i g . 4 : P h o r e d o x , 3 - s t a g e
B a r d e n p h o , J o h a n n e s b u r g , U n i v e r s i ty o f C a p e T o w n
( U C T ) , a n d th e m o d i f i ed U C T ( M U C T ) c o n f i g u r-
a t i o n . D a t a w e r e r e p o r t e d f o r e a c h s y s t em f o r a
n u m b e r o f d if f e r e n t b a t c h e s o f i n f l u en t w a s t e w a t e r
( d e n o t e d b y a l e t t e r a p p e n d e d t o t h e s y s t e m n u m b e r
i n T a b l e 4 ) . I n a d d i t i o n , W e n t z e l e t a l (1 9 8 9 ) r e -
p o r t e d d a t a f o r f o u r d i f f e r e n t l a b - s c a l e e n h a n c e d
c u l t u r e B E P R s y s t e m s w i t h a c e t a t e a s i n f l u e n t . T h e
e n h a n c e d c u l t u r e s ( g r e a t e r t h a n 9 0 p o l y P o r g a n -
i s m s ) w e r e d e v e l o p e d u s in g m o d i f i e d B a r d e n p h o a n d
U C T s y s t e m s o p e r a t e d a t s l u d g e a g e s o f 7. 5, 1 0 a n d
2 0 d a y s .
P H O R E D O XNO)S Y S T E M
r
3-STAGEA R D E N P H OY S T E M
U T
S Y S T E M
I I I 1
1 I I
MODIFIEDC T Y S T E M
JOHANNESBURG]Y STEM ~ O
L E G E N D ~
A E R O B IC E A C T O R
A N O X lC E A C T O R
A N A E R O B IC E A C T O ~
Fig . 4 . NDBEPR sys tem configura t ions fo r which Wentze l e t a l (1989 , 1990) reported da ta .
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638 P S BARKER and P L DOLD
Mass balances for COD and N were conducted on
the data sets for which all the relevant data were
reported; in certain instances VSS data were omitted
and it was not possible to calculate balances. In
certain of the r emaining cases for the mixed culture
systems treating municipal wastewater, the validity
of the data was questionable. This was generally
reflected by very poor N balances; for example,
balances of 200% output N compared to input N.
Those data sets were not considered here. Table 4
presents results of COD and N mass balance calcu-
lations for the mixed culture systems with N balances
between 90 and 110%, as well as the enhanced culture
systems. The following observations are noted:
COD balances on the BEPR systems treating
municipal wastewater are generally substan-
tially lower than those for the non-BEPR
systems described earlier (some systems show
balances below 70%)
the average of the COD balances for the mixed
culture BEPR systems with municipal waste-
water as influent is only 78%
COD balances for the enhanced culture sys-
tems with acetate as influen t are all close to the
average of 91% for these systems
N balances for both the mixed culture and the
enhanced culture BEPR systems are close to
100%
These results suggest that the presence of an
anaerobic zone may lead to an appreciable reduction
in the calculated COD balance for systems with a
fermentable substra te (such as domestic wastewater).
DISCUSSION
The following points summarize the results for
COD and N balances conducted on the four types of
activated sludge system:
COD balances on the completely aerobic sys-
tems are close to 100%
COD balances on the anoxic only systems
range from 95 to 85%, possibly with a de-
creasing trend as the sludge age increases
the anoxic-aerobic systems which did not ex-
hibit anaerobic conditions show COD bal-
ances close to 100%, while those in which the
unaerated reactor nitrate concentration
dropped to zero have balances less than 80%
the average of the COD balances for the mixed
culture BEPR systems with municipal waste-
water as influent is only 78%. The average
for the enhanced culture BEPR systems with
acetate as influent is 91/'o
nitrogen balances for all systems are close
to 100%. This would indicate that nitrogen
loss th rough denitrification under aerobic con-
ditions was n ot significant for these systems.
The mos t significant finding is that for systems
incorporating anaerobic zones (i.e. BEPR systems)
the COD balances, averaging less than 80%, do not
account for a substantial portion of the influent
COD.
This apparent loss of COD in BEPR systems
with anaerob ic zone has been reported previously, for
example, by Burke e t a l (1986) in a study on short
Table 4. COD and nitrogen mas s balances for BEPR systems [data from Wentzel
e t a l
(1989, 1990)]
Sludge age N balance COD balance
System Type* (d) ( ) ( )
S y s t e m s w i t h m u n i c i p a l w a s t e w a t e r a s i n f l u e n t
1 Phoredox 3 93.9 79.4
1b Phor edox 3 109.8 64.6
2a Phored ox 4 95.7 85.6
5a Jhb 5 103.3 81.3
6a UC T 6 97.5 67.0
8a UC T 8 99.2 89.7
10b UC T 8 110.0 74. I
1 a UC T 10 98.0 70.0
16a MU CT 15 95.2 85.5
18a MU CT 15 98.6 83.2
19b MU CT 15 96.8 74.2
19c MU CT 15 94.6 85.4
22a MU CT 20 103.9 65.3
22b MU CT 20 97.6 77.8
23a MU CT 20 95. I 73.2
23b MU CT 20 92.1 86.9
23c MU CT 20 100.3 87.6
24a MU CT 20 100.2 60.7
24c M UC T 20 92.4 80.0
24d MU CT 20 93.4 84.3
26a MU CT 21 102.8 85.2
Averages 98.6 78.1
E n h a n c e d c u l t u r e s y s t e m s w i t h a c e t a t e a s i n f l u e n t
1 Bar den pho 20 103.6 88.9
2 UC T 10 103.1 91.1
3 Barde npbo I 0 88.6 90.4
4 Barde npho 7.5 95.9 92.3
Aver ages 97.8 90.7
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COD and nitrogen mass balances
639
s l u d g e ag e B E P R s y s te m s . B o r d a c s a n d T r a c y ( 1 9 88 )
o b s e r v e d t h a t t h e p r e s e n c e o f a n a n a e r o b i c z o n e
l e a d s t o a s m u c h a s a 3 0 % r e d u c t i o n i n o x y g e n
r e q u i r e m e n t s c o m p a r e d t o a c o n v e n t i o n a l a e r o b i c
p r o c e s s . H o w e v e r , t h e s t u d y w a s b a s e d o n t h e a s -
s u m p t i o n t h a t t h e r e d u c t i o n in o x y g e n c o n s u m p t i o n
i s d u e t o t h e r e t e n t i o n o f o r g a n i c s t o r a g e p r o d u c t s
( s u c h a s P H B ) b y t h e p o l y P o r g a n i s m s , a n d C O D
m e a s u r e m e n t s w e r e n o t m a d e . D o l d ( 1 9 9 0 ) n o t e d
t h a t , u n l e s s C O D l o ss a s s o c i a t e d w i t h a n a e r o b i c
f e r m e n t a t i o n w a s t a k e n i n t o a c c o u n t , B E P R a c t i -
v a t e d s l u d g e m o d e l s o v e r p r e d i c t b o t h o x y g e n c o n -
s u m p t i o n r a t e s in a e r a t e d z o n e s o f B E P R s y s te m s a n d
v o l a t i le s u s p e n d e d s o l i d s p r o d u c t i o n .
A r e q u i s i t e f o r B E P R i s t h e p r e s e n c e o f a n
a n a e r o b i c z o n e w h i c h a l l o w s th e p o l y P o r g a n i s m s t o
s e q u e s t e r s h o r t c h a i n f a t t y a c i d s ( S C F A ) s u c h a s
a c e t a te . T h e s e S C F A s a r e s t o r e d b y t h e o r g a n i s m s i n
t h e f o r m o f P H B u n t i l a n e l e c t r o n a c c e p t o r is a v a i l -
a b l e . T h e s t o r e d P H B i s t h e n u s e d f o r g r o w t h a n d P
u p t a k e .
G e n e r a l l y t h e i n f l u e n t w a s t e c o n t a i n s o n l y a v e r y
l o w c o n c e n t r a t i o n o f S C F A ; t h e r e f o r e t h e S C F A
n e c e s s a r y f o r B E P R m u s t b e p r o d u c e d w i t h i n t h e
s y s te m , l i k e ly in t h e a n a e r o b i c r e a c t o r t h r o u g h a
f e r m e n t a t iv e p ro c e s s . T h e p r o d u c t i o n o f S C F A u n d e r
a n a e r o b i c c o n d i t i o n s h a s b e e n o b s e r v e d b y M e g a n c k
e t a l . ( 19 8 5) . I t w a s n o t e d t h a t s i g n i f i c a n t n u m b e r s
o f t h e b a c t e r i a
A e r o m o n a s h y d r o p h i l a
a f a c u l t a t i v e
o r g a n i s m c a p a b l e o f u t i l i zi n g s o m e s u g a r s a n d a l c o -
h o l s u n d e r a n a e r o b i c c o n d i t i o n s , w e r e p r e s e n t i n
s l u d g e f r o m a l a b - s c a l e B E P R s y s te m . P r o d u c t i o n
o f S C F A i n t h e a n a e r o b i c z o n e o f B E P R s y s te m s
p r e s u m a b l y r e s u lt s f ro m t h e f e r m e n t a t i o n o f th e
c o m p l e x r e a d i l y b i o d e g r a d a b l e C O D i n t h e i n fl u e n t
( i. e. t h e S C F A a r e p r o d u c e d v i a t h e o x i d a t i o n o f t h e
i n fl u e n t c o m p l e x r e a d i l y b i o d e g r a d a b l e C O D u s i n g
a n i n t e r n a l l y s u p p l i e d o r g a n i c c o m p o u n d a s t h e
o x i d i z i n g a g e n t ). T h i s h y p o t h e s i s i s s u p p o r t e d b y t h e
w o r k o f B o r d a c s a n d C h i e s a ( 19 8 9 ) w h o u s e d r a d i o -
l a b e l l e d s u b s t r a t e s ( g l u c o s e a n d a c e t a t e ) t o t r a c k t h e
c a r b o n f l o w i n p h o s p h o r u s a c c u m u l a t i n g c u l t u r e s .
R e s u l t s i n d i c a t e d t h a t a g r e a t e r p e r c e n t a g e o f t h e
l a b e l le d c a r b o n w a s c o n v e r t e d t o c a r b o n d i o x i d e
u n d e r a n a e r o b i c c o n d i t i o n s w h e n g l u c o s e w a s t h e
l a b e l le d s u b s t ra t e c o m p a r e d t o a c e t a t e ( 1 2 a n d 2 % ,
r espec t ive ly) .
T h e l o s s o f C O D i n B E P R s y s te m s m o s t l i k e ly
i s a s so c i a t e d w i t h t h e f e r m e n t a t i o n p r o c e s s o c c u r r i n g
i n t h e a n a e r o b i c z o n e . H o w e v e r , a f u ll u n d e r s t a n d i n g
o f t h e f e r m e n t a t i o n b e h a v i o u r d o e s n o t e x i s t . I t h a s
b e e n s u g g e s t ed t h a t t h e C O D l o s s i s d u e t o r e le a s e
o f g a s e o u s f e r m e n t a t i o n p r o d u c t s ( e x c l u d i n g c a r b o n
d i o x i d e w h i c h d o e s n o t h a v e a C O D ) .
B u r k e
e t a l .
( 1 9 8 6 ) s u g g e s t e d t h a t g e n e r a t i o n o f
h y d r o g e n g a s o c c u r s d u r i n g t h e a c i d o g e n e s i s p r o c e s s
i n th e a n a e r o b i c r e a c t o r . M a n y f a c u l ta t i v e o r g a n i s m s
h a v e b e e n d o c u m e n t e d t h a t a r e c a p a b l e o f f e r m e n t i n g
g l u c o s e t o p r o d u c e h y d r o g e n a n d c a r b o n d i o x i d e
g a s e s ( S t a n i e r
e t a l .
1 9 7 6 ) . A m o n g t h e s e a r e s o m e
s p e c i e s o f t h e g e n u s
A e r o m o n a s
m e n t i o n e d e a r l i e r a s
o n e o f th e o r g a n i s m s f o u n d t o o c c u r i n s i g n i fi c a n t
n u m b e r s i n B E P R p i l o t p l a n t s (M e g a n c k e t a l . 1985;
M a l n o u e t
a l .
1984 .
A p o s s i b l e a l t e r n a ti v e t o h y d r o g e n a s a g a s e o u s
C O D l o ss w o u l d b e g e n e r a t i o n o f m e t h a n e d u r i n g
f e r m e n t a t i o n . F o r t h e s y s t e m s i n v e s t i g a t e d i n t h i s
s t u d y , t h e o re t i c a l ly m e t h a n e p r o d u c t i o n s h o u l d n o t
h a v e b e e n p o s s i b l e d u e b o t h t o t h e t e m p e r a t u r e a t
w h i c h t h e s e p i l o t p l a n t s y s t e m s w e r e o p e r a t e d ( 2 0 C ) ,
a n d t h e f a c t t h a t m e t h a n o g e n i c b a c t e r i a a r e o b l i g a t e
a n a e r o b e s ( S t a n i e r , 1 9 76 ) a n d w o u l d n o t l i k e l y s u r -
v i v e t h e a n a e r o b i c / a e r o b i c s e q u e n c i n g . H o w e v e r , t h i s
p o s s i b i l it y s h o u l d n o t b e i g n o r e d a s m e t h a n o g e n i c
b a c t e r i a c a p a b l e o f t o l e r a t i n g l o w o x y g e n c o n c e n -
t r a t i o n s h av e b ee n d o c u m e n t e d ( G r a d y a n d L i m ,
1980).
A n a l t e r n a t i v e t h e o r y t o e x p l a i n t h e s i g n i f i c a n t
d i s a p p e a r a n c e o f C O D i n B E P R s y s t e m s i s t h e
h y p o t h e s i s t h a t f e r m e n t a t i o n i n t h e a n a e r o b i c r e a c t o r
r e su l t s i n t h e p r o d u c t i o n o f v o l a t il e c o m p o u n d s ,
w h i c h a r e t h e n r e l e a s e d f r o m t h e s y s t e m u n d e r
a e r o b i c c o n d i t io n s . T h e p r o d u c t i o n o f v o l a t il e c o m -
p o u n d s ( s u c h a s e t h a n o l , a c e t i c a c i d a n d o t h e r
v o l a t il e f a tt y a c i d s ) u n d e r o x y g e n l i m i t e d c o n d i t i o n s
h a s b e e n d o c u m e n t e d f o r a n u m b e r o f f a c u lt a t iv e
o r g a n i s m s ( S t a n i er e t a l . 1976; V ol lbr echt , 1982) . A
r e c e n t s t u d y b y W a b l e a n d R a n d a l l ( 1 9 9 2) p r o p o s e d
t h a t t h i s i s a m o r e p r o b a b l e m e c h a n i s m f o r C O D
l o ss th a n t h a t o f h y d r o g e n o r m e t h a n e g e n e r a t i o n .
H o w e v e r , t h e v o l a t i l i z a ti o n m e c h a n i s m s e e m s u n -
l i k e l y a s t h e s e r e a d i l y b i o d e g r a d a b l e c o m p o n e n t s
s h o u l d b e r e m o v e d f r o m s o l u t i o n p r i o r to t h e a e r a t e d
z o n e .
A s i d e fr o m t h e o b s e r v a t i o n s o n B E P R s y s-
t e m s t r e a t i n g m u n i c i p a l w a s t e w a t e r , t h i s s t u d y h a s
i d e n t i f ie d t w o a d d i t i o n a l a s p e c t s w h i c h r e q u i r e
f u r t h e r r e s e a r c h :
r e s u l t s f r o m a n o x i c o n l y s y s t e m s i n d i c a t e
t h a t C O D l o s s o c c u r s t o a l i m i t e d e x t e n t
i n t h e s e s y s t e m s ; t h i s m a y a l s o b e a s s o c i a t e d
w i t h f e r m e n t a t i o n . G e n e r a l l y t h e p e r c e p t i o n
i s t h a t f e r m e n t a t i o n s h o u l d n o t o c c u r i n
a n o x i c z o n e s o f a c t i v a t e d s l u d g e s y s t e m s .
H o w e v e r , f e r m e n t a t i o n h a s b e e n o b s e r v e d
i n t h e p r e s e n c e o f n i t r a t e i n p u r e c u l t u r e
s t u d i es ( H a d j i p e t r o u a n d S t o u t h a m e r , 1 96 5;
S t o u t h a m e r a n d B e t t e n h a u s e n , 1 9 7 2)
i n t h e e n h a n c e d c u l t u r e s y s t e m s w i t h a c e t a t e a s
i n fl u e n t t h e C O D b a l a n c e s d o n o t a c c o u n t f o r
a p p r o x . 1 0 % o f t h e i n p u t C O D . F e r m e n t a t i o n
i n t h es e s y s te m s s h o u l d b e m i n i m a l . T h e C O D
l o s s p o s s i b l y i s a s s o c i a t e d w i t h t h e p r o c e s s
o f P H B f o r m a t i o n .
ON LUSIONS
T h e r e s u l t s o f t h i s s t u d y s u g g e s t t h a t w h i l e
g o o d C O D b a l a n c e s a r e t o b e e x p e c t ed i n a e ro b i c
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640 P. S. BARKER an d P. L. DOLD
a n d a e r o b i c - a n o x i c s y s t e m s , s y s t e m s i n c o r p o r a t i n g
a n a e r o b i c z o n e s ( i . e . B E P R s y s t e m s ) t e n d t o e x h i b i t
l o w C O D b a l a n c e s ( le ss t h a n 8 0 % ) . T h i s l o s s o f
C O D a p p a r e n t l y is a s s o c i a t e d w i t h th e f e r m e n t a t i o n
p r o c e s s e s o c c u r r i n g i n t h e a n a e r o b i c z o n e o f B E P R
s y s t em s t r e a t i n g m u n i c i p a l w a s t e w a t e r . W h e t h e r t h i s
C O D l o ss is a d i r e ct r e s u lt o f f e r m e n t a t i o n ( t h r o u g h
t h e g e n e r a t i o n o f g a s w h i c h e v o lv e s d u r i n g t h e a c t u a l
f e r m e n t a t i o n p r o c e s s ) , o r a n i n d i r e c t r e s u l t ( t h r o u g h
t h e p r o d u c t i o n o f v ol a ti le c o m p o u n d s w h i c h a r e
r e l e a s e d f r o m t h e s y s t e m u n d e r a e r a t e d c o n d i t i o n s ) ,
r e m a i n s t o b e d e t e r m i n e d .
I r r e sp e c t i v e o f t h e r e a s o n ( s ) fo r t h e C O D l o s s ,
t h is p h e n o m e n o n h a s s i g n if i c a nt i m p l i c a t i o n s w i t h
r e g a r d s t o r e d u c e d a e r a t i o n c o s t s a n d s l u d g e p r o -
d u c t i o n i n B E P R v e r s u s c o n v e n t i o n a l a c t i v a t e d
s l u d g e s y s t e m s . I f t h e c a u s e s o f t h is l o ss o f C O D c a n
b e d e t e r m i n e d , i t m a y b e p o s s i b le t o d e s i g n a n d / o r
o p e r a t e s y s t e m s s o a s t o m a x i m i z e C O D l o s s t h e r e b y
r e d u c i n g t h e c o s t o f a e r a t i o n a n d s l u d g e t r e a t m e n t /
d i s p o s a l .
A c k n o w l e d g e m e n t - - T h i s s t u d y f a r m e d p a r t o f t h e a c t iv i t ie s
o f t h e N a t u r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h C o u n c i l
( N S E R C ) / W a s t e w a t e r T e c h n o l o g y C e n t r e I n d u s t r i al C h a i r
i n E n v i r o n m e n t a l S y s t e m s E n g i n e e r i n g . T h e s u p p o r t o f t h e
C h a i r s p o n s o r s i s a c k n o w l e d g e d .
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Removal Ac t iva ted S ludge Processes . W a t e r R e s e a r c h
C o m m i s s i o n o f S o u t h A f r i c a .
P P E N D I X
COD and N Balance Calculat ions
1) Nomenc la ture
Q = average in f luent f lowra te ( l /d )
q = average was tage f lowra te ( l /d )
$ 1 , = t o t a l i n f l u e n t C O D ( m g C O D / l )
ST~ = t o t a l e f f l u en t C O D ( m g C O D / l )
f c v = C O D / V S S r a t i o ( m g C O D / r a g V S S )
X v = m i x e d l i q u o r v o l a t il e s u s p e n d e d s o l id s o f
was te s t ream (mg VSS/I )
O T = t o t a l o x y g e n u t i l iz a t i o n r a t e ( m g O / l / h )
M O ~ = m a s s o f o x y g e n c o n s u m e d f o r n i t r i fi c a t i o n
( r a g O / d )
V~,~r= volume of aerobic reac tor ( l i t res )
NTi = average in f luent TKN (mg N/I )
NT~
= a v e r a g e e f f l u e n t T K N ( m g N / l )
NNe = averag e e f f luen t n i t r a te (mg NO3-N/I
NN.,~r = av erag e ae rob ic n i tra te (m g N O3-N/I)
N N . . . .. = averag e an oxic n i t ra te ( rag NO 3-N/I )
NN . .. .. . = av erage ana erob ic n i t ra te (m g N O3-N/I )
. [~ = n i t rog en f rac t ion of was te s ludg e (mg N / l )
s = se t t l e r under f low recyc le ra t io wi th respec t to
inf luent f lowra te
r = anoxic mixed l iquor recyc le ra t io
a = n i t r i f i ed /aerobic mixed l iquor recycle ra t io
Mdcn,~ . .. .. . = m ass of ni tr ate den itr if ied in the ano xic zon e
( r a g N / d )
Md~.. . .. .. . = mass of n i t ra te deni t r i f i ed in the ana erob ic
z o n e ( r a g N / d )
Ma~.,~,T = t o t a l m a s s o f n i t r a t e d e n i t r i fi e d i n t h e s y s t em
( m g N / d )
MCOD.~M= m a s s o f C O D i n t h e s y s t e m e f f l u en t ( r ag
C O D / d )
Mcoo,,,,,
= m a s s o f C O D i n t h e w a s t a g e s t r e a m ( r a g
C O D / d )
McoD ae r =
m a s s o f C O D o x i d i z e d u n d e r a e r o b i c c o n -
d i t io n s ( m g C O D / d )
MC'OD.d~.. = mass of C OD oxid ized th rou gh deni t r i f i ca t io n
( m g C O D / d )
McoD.o,,~d
= t o t a l m a s s o f C O D o x i d iz e d i n t h e sy s t e m ( ra g
C O D / d )
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C O D a n d n i t r o g e n m a s s b a l a n c e s
641
M N.N ~ = m a s s o f n i t r a t e - n i t r o g e n i n t h e s y s t e m e f f l u e n t
( m g N / d )
MN,T~ = m a s s o f T K N i n t h e s y s t e m e f f lu e n t ( r ag N / d )
M N . . . . . =
m a s s o f n i t r o g e n i n t h e w a s t a g e s t re a m ( m g
N / d ) .
1 1) D e n i t r i f i c a t i o n c a l c u l a t i o n s
I n n i t r i f y i n g s y s t e m s w i t h u n a e r a t e d z o n e s , i n o r d e r t o
p e r f o r m b o t h C O D a n d N b a l a n c e s , t h e m a s s o f n i t r a t e
w h i c h i s d e n it r i fi e d i n e a ch o f t h e u n a e r a t e d r e a c t o r s m u s t
b e d e t e r m i n e d f r o m m a s s b a l a n c e o n n i t r a t e . R e f e r r i n g t o
F i g . A 1 , f o r a U C T t y p e s y s t e m , t h e m a s s o f n i t r a t e e n t e r i n g
t h e a n o x i c r e a c t o r p e r d a y i s g i v e n b y :
i n p u t n i t r a t e = aQ NN. ae r + sQ Nr~ ~ + (1 + r ) Q N N . . . .. .
S i m i l a r l y , t h e m a s s o f n i t r a t e l e a v i n g t h e a n o x i c r e a c t o r i s
g i v e n b y :
o u t p u t n i t r a t e = r Q N N . . .. + (1 + s + a ) Q N N . . ..
= ( l + r + s + a ) Q N N . . . .
T h e m a s s o f n i t r a t e d e n i t r i f i e d p e r d a y ( M d ~ , , . . . . . ) i s e q u a l
t o t h e d i f f e re n c e b e tw e e n t h e i n p u t a n d o u t p u t n i t ra t e :
Mdeni . . . . . = aQ NN. ae 4 - sQ NNe 4 - (1 4- r ) Q N N . . . . .
- l + r + s + a ) Q N N . .. .. ( A I )
A s a s m a l l q u a n t i t y o f n i t r a t e w i ll o f t e n b e i n a d v e r t e n t l y
r e c y c l e d t o t h e a n a e r o b i c r e a c t o r , a n y d e n i t r i f i c a t i o n
o c c u r r i n g i n t h e a n a e r o b i c r e a c t o r s h o u l d a l s o b e i n c l u d e d
i n m a s s b a l a n c e c a l c u l a t io n s . F o r a U C T s y s t e m w i t h o n e
a n a e r o b i c r e a c t o r ( a s s u m i n g n o n i t r a t e i n t h e i n f l u en t ) , th i s
i s g iven by :
M@ni .. . . . . = r Q N N . . . . . - - (1 + r ) Q N N . . . . . (A2)
T h e t o t a l m a s s o f n i t r a t e d e n i tr i f ie d p e r d a y i n t h e
s y s t e m i s t h e r e f o r e t h e s u m o f t h e m a s s d e n i t r i f i e d i n t h e
a n a e r o b i c r e a c t o r , a n d t h e m a s s d e n i t r i f i e d i n t h e a n o x i c
reac to r , i . e . :
Mdenit.T = Mdeni ...... 4- Mdeni .... .. (A 3)
I 1 1) C O D b a l a n c e c a h : u l a t i o n s
T o p e r f o r m a C O D b a l a n c e o n a s y s t e m , i t i s n e c e s s a ry
t o e s t i m a t e t h e m a s s o f C O D i n t h e ef f lu e n t, t h e C O D o f t h e
w a s t e s l ud g e , a n d t h e a m o u n t o f C O D o x i d i ze d . I f t h e
e f fl u e n t C O D i s k n o w n , t h e m a s s o f C O D i n t h e e ff l ue n t i s
s i m p l y :
McoD.e t a = QST e ( A 4 )
S i m i l a rl y , i f t h e v o l a t i le s u s p e n d e d s o l i ds c o n c e n t r a t i o n
o f t h e w a s t e s l u d g e ( X v ) is k n o w n , t h e n b y a s s u m i n g a
v a lu e 2 o f 1.4 8 m g C O D / m g V S S f o r f c v , t h e m a s s o f C O D
w a s t e d i s g i v e n b y :
MCOD..a = q X v f c v ( A 5 )
I d e a l l y , t h e C O D / V S S r a t i o f c v s h o u l d b e d e t e r m i n e d
e x p e r i m e n t a l l y f o r a p a r t i c u l a r s l u dg e ; h o w e v e r , t h e v a l u e o f
1 . 4 8 m g C O D / m g V S S h a s be e n sh o w n t o b e a g o o d
a p p r o x i m a t i o n o v e r a r a n g e o f s lu d g e ag e s a n d w a s t e w a t e r
c h a r a c t e r i s t i c s ( S c h r o e t e r e t a l . , 1982).
I n a p u r e ly a e r o b i c s y s te m , t h e a m o u n t o f C O D o x i d i z e d
i s d e t e r m i n e d f r o m t h e o x y g e n u t i l iz a t i on r a t e , a f t e r d e d u c t -
i n g t h e o x y g e n r e q u i r e d f o r n i t r i f i c a t i o n . T h i s c a n b e d o n e
b y a s s u m i n g t h e s o m e w h a t s i m p l i f i e d r e l a t i o n s h i p :
N H ~ - + 2 0 2 - - * N O 3 + 2 H + + H 2 0 ( A 6 )
E q u a t i o n ( A 6 ) i m p l ie s th a t 1 m o l o f a m m o n i a r e q u i r e s 2 m o l
o f o x y g e n t o f o r m 1 m o l o f n i t r a t e . T a k i n g m o l e c u l a r
w e i g h t s i n to a c c o u n t , t h i s im p l i e s t h a t 1 4 m g N H 4 - N r e q u i re
6 4 m g o x y g e n , o r e q u i v a l e n t l y , i f x m g 1 o f N O ~ - N a r e
f o r m e d , t h e n 4 . 5 7 . x m g O / 1 a r e c o n s u m e d . I n a p u r e l y
a e r o b i c s y s t e m , t h e m a s s o f n i t r a t e f o r m e d i s g i v e n b y t h e
p r o d u c t o f t h e i n f l u e n t f l o w r a t e a n d t h e e f f lu e n t n i t r a t e
c o n c e n t r a t i o n ( N Ne ). F o r s y s t e m s i n c o r p o r a t i n g d e n i tr i f ic a -
t i o n , t h e m a s s o f n i t r a t e f o r m e d i s g i v en b y :
m a s s n i t r a t e f o r m e d = M a e . , . v + Q N N e
T h u s t h e m a s s o f o x y g e n c o n s u m e d d u e t o n i t r i f ic a t i o n
M O N ) i s g iven by :
M O N = (Md~.u .T + QNNe)4.57 (A7 )
T h e r e f o r e t h e t o ta l m a s s o f C O D o x i d i z e d p e r d a y u n d e r
a e r o b i c c o n d i t i o n s ( M c o D . . . r ) , i s g i v e n b y :
McOD .ae = O T I4~er24 - - M O N
= O T Vaer24 - (Mdenit. + QN Ne)4.57 (A 8)
A l t h o u g h e q u a t i o n ( A 6 ) r e p r e s e n t s a s im p l i f i c a ti o n o f
t h e a c t u a l r e a c t i o n s m e d i a t e d b y t h e n i t r i f y i n g o r g a n i s m s
N i t r o s o m o n a s a n d N i t r o b a c t e r , t h e e r r o r i n t r o d u c e d b y
t h i s s i m p l i f i c a t i o n s h o u l d b e m i n i m a l ( G r a d y a n d L i m ,
1980).
I n s y s t e m s i n c o r p o r a t i n g u n a e r a t e d z o n e s , s u c h a s t h e
U C T d e s i g n , t h e m a s s o f C O D o x i d i ze d t h r o u g h d e n i t r i fi c a -
t i o n m u s t a l s o b e t a k e n i n t o a c c o u n t i n t h e C O D b a l a n c e .
T h i s i s d o n e b y u s i n g t h e e q u i v a l e n c e f a c t o r o f 2 . 8 6 o u t l i n e d
ea r l i e r .
T h u s i f
M d e n i t . T
i s t h e t o t a l m a s s o f n i t r a t e d e n i t r i f i e d p e r
d a y , t h e n t h e t o t a l C O D c o n s u m e d t h r o u g h d e n i t r i f i c a t i o n
is g iven by :
McoD,deni = 2 .86Mdenit, r ( A 9 )
T h e r e f o r e t h e t o ta l a m o u n t o f C O D o x i d i z ed in a s y s te m
i n c o r p o r a t i n g b o t h n i t r i f i c a t i o n a n d d e n i t r i f i c a t i o n , i s g i v e n
by :
mcoo.oxid = McoD.a~.i, + MCOD ...
= 2.86Macit.+ + O+ Vde,24 -- (Mde. i t. r + QNN ~)4-57 (A 10)
T h e to t a l a m o u n t o f ' o u t p u t C O D c a n t h e re f o r e b e
de t e rm ine d f rom the sum o f M coD.et M coD. .~ and M COD.o~ ,d
u s i n g e q u a t i o n s ( A 4 ) , (A 5 ) a n d ( A I 0 ) , a s w e l l a s t h e e s t i m a t e
f o r M d .~ .T o b t a i n e d f r o m m a s s b a l a n c e s o n n i t r a t e a r o u n d
t h e u n a e r a t e d r e a c t o r s ( as d e s c r i b e d e a r l ie r f o r t h e U C T
system), i .e . :
o u t p u t C O D
= M c o D , e t ~ + M c o D . w a s - - M , o O .o x ~ d
(A 11 )
r .Q
a Q
T o I
F i g . A I . U C T s y s t e m 6a c o n f i g u r a ti o n [ f r om W e n t z e l e t a l . (1990)].
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P. S. BARKERa n d P . L . D O L D
Table A1. Steady-s tate data for the four-reactor UCT System 6a [data f rom Wentzel et aL (1990)]
Reactor volum es ( l it res ) Anae robic 2.0 lnf luent f lowrate ( l /d) 25
Anox ic 2.0 r recycle rat io 1
Aerobic 2.0 s recycle ratio I
Aerobic 2.0 a recycle ratio I
Wa stag e rate (I/d) 1.174
Param eter lnf luent Ana erobic Anox ic Aerobic Aerobic Eff luent
C O D (mg/l) 510 . . . . 40
T K N (m g N /l) 42 . . . . 4
NO 3 (mg N/I) 0.0 0.5 1.5 9.4 10.6 10.5
O U R ( m g / l/ h ) - - - - 7 8 4 3 - -
VSS (m g/I) . . . . 2100
T h e to t a l m a s s p e r d a y o f i n p u t C O D i s g i v en b y
t h e p r o d u c t o f th e i n f l u e n t f l o w r a t e a n d t h e i n f l u e n t C O D ,
i.e.:
i n p u t C O D = QST, (A 12)
T h u s t h e % C O D b a l a n c e i s g i v e n a s:
% C O D b a la n c e = ( o u t p u t C O D ~ 10 0
\
i np u t C O D
IV) Nitrogen balance calculat ions
T o p e r f o r m a n i t r o g e n b a l a n c e o n a s y s t e m , it is n e c e s s a r y
t o e s t i m a t e t h e m a s s o f n i t r a t e i n t h e e f f l u e n t , t h e m a s s o f
e f f lu e n t T K N ( u n f il t e re d ) , t h e m a s s o f T K N i n t h e w a s t e
s l u d g e a n d t h e n i t r o g e n l o s s t h r o u g h d e n i t r i f i c a t i o n .
I n t e r m s o f t h e v a r i a b l e s d e f i n e d e a rl i e r, t h e m a s s o f
n i t r a t e i n t h e e f f l u e n t ( M N . N E ) i s g i v e n b y :
MN,Ne = QNN~ (A 13)
S i m i l a r l y , t h e m a s s o f T K N i n t h e e f f l u e n t ( M N .T ,) i s g i v e n
by :
MN.T~ = Q N w ( A 1 4 )
T o e s t i m a t e t h e m a s s o f N i n t h e w a s t e s l u d g e , a v a l u e f o r
f N , t h e n i t r o g e n f r a c t i o n o f t h e s l u d g e , m u s t b e a s s u m e d .
E x p e r i m e n t a l e v i d e n c e s u g g e s ts a v a l u e o f 0 .1 m g N / m g V S S
i s r e a s o n a b l e o v e r a r a n g e o f s l u d g e a g e s ( W R C , 1 98 4) ;
h o w e v e r , i d e al l y f N s h o u l d b e d e t e r m i n e d e x p e r i m e n t a l l y
f o r a p a r t i c u l a r s y s t e m a n d s e t o f o p e r a t i n g p a r a m e t e r s .
G i v e n a v a l u e f o r f N , t h e m a s s o f N i n t h e w a s t e s l u d g e
( M N ,w a s) i s c a l c u l a t e d f r o m :
M N . . . = qXv.fN (A 15)
I n S e c t i o n I I t h e m e t h o d w a s o u t l i n e d f o r e s t i m a t i n g
t h e m a s s o f N O a - N w h i c h i s d en i t ri f i ed i n t h e u n a e r a t e d
r e a c t o r s . F o r a U C T s y s t e m , M d ,n ,.T i s c a l c u l a t e d u s i n g
e q u a t i o n s ( A 1 ) , ( A 2 ) a n d ( A 3 ) .
T h e t o t a l o u t p u t N i s t h e r e f o r e t h e s u m o f M N .N e , MN,T,
M N . . . an d Md.i t,T , w h i c h c a n b e c a l c u l a t e d u s i n g e q u a t i o n s
( A l 3 ) , ( A 1 4 ) a n d ( A l 5 ) , a s w e l l a s t h e e s t i m a t e f o r M d ~ m t . r
o b t a i n e d f r o m a m a s s b a l a n c e o n n i t r a t e , i. e. :
o u t p u t N
= M N . N e M N , T e - {- M N . w a s M d e n i t . T
( A I 6 )
T h e t o t a l m a s s p e r d a y o f i n p u t N i s g i v e n b y t h e p r o d u c t
o f t h e i n fl u e n t f l ow r a t e a n d t h e i n f lu e n t T K N ( a s s u m i n g
zero n i t r a t e i n the in f luen t ) , i . e . :
i n p u t N =
QNTi
(A 17)
T h e r e f o r e t h e % N b a l a n c e i s g i v e n as :
( o u t p u t N )
% N b a l a n c e = i n p u t N 1 00
U s i n g S y s t e m 6 a f r o m W e n t z e l
et al.
( 1 9 9 0 ) a s a n
e x a m p l e , t h e f o l lo w i n g s e c t i o n o u t l i n e s th e m a s s b a l a n c e
p r o c e d u r e .
V) COD a nd N ba lance s - -exa mp le ca lcu la t ions
T h e d a t a n e c e s s a r y f o r p e r f o r m i n g N a n d C O D b a l a n c e s
f o r S y s t e m 6 a a r e l i s te d i n T a b l e A 1 . S y s t e m 6 a i s a U C T
c o n f i g u r a t i o n w i t h t w o a e r o b i c r e a c t o r s , o n e a n o x i c a n d o n e
a n a e r o b i c r e a c t o r a s s h o w n .
Denitrification calculations. T h e m a s s o f N O 3 - N d e n i -
t ri f le d i n t h e a n o x i c r e a c t o r i s g i v e n b y e q u a t i o n ( A I ) :
M d e n i t . . . . . = 4 0 2 .5 m g N / d
T h e m a s s o f N O 3 - N d e n i t r i fi e d i n th e a n a e r o b i c r e a c t o r i s
o b t a i n e d f r o m e q u a t i o n ( A 2 ) :
M d e n i t . . . . . . = 1 2.5 m g N / d
T h e t o t a l m a s s o f N O 3 - N d e n i t r i fl e d i n t h e s y s t e m is th e s u m
of Md~nit . . . . a nd Md~ni . . . .. e qu a t i on (A3) :
MdeniLT = 415 m g N /d
COD balance calculat ions. F r o m e q u a t i o n ( A 4 ) , t h e m a s s
o f C O D i n t h e e f f l u e n t i s:
M co D .e m = 1 0 0 0 m g C O D / d
T h e m a s s o f C O D i n t h e w a s te s l u d g e is c al c u l at e d u s i n g
e q u a t i o n ( A 5 ) :
M c oD .w ~ = 3 6 4 9 m g C O D / d
U s i n g e q u a t i o n ( A I 0 ) , a n d t h e v a l u e f o r M d ~ n,,T c a l c u l a t e d
a b o v e , t h e m a s s o f C O D o x i d i z e d is :
M c oD .o xid = 3 8 9 9 m g C O D / d
T h e t o t a l o u t p u t C O D i s t h e s u m o f t h e a b o v e t h re e t e r m s
[or equa t ion (11) ] :
o u t p u t C O D = 8 5 48 m g C O D / d
A p p l y i n g e q u a t i o n ( 1 2 ) g i v e s :
i n p u t C O D = 1 2 ,7 5 0 m g C O D / d
T h e r e f o r e , t h e % C O D b a l a n c e i s g i v e n b y :
% C O D b a l an c e = ( o u t p u t C O D / i n p u t C O D ) ' 1 0 0
= ( 8 5 4 8 / 1 2 , 7 5 0 ) ' 1 0 0
= 6 7 . 0 %
T h e r e s u l t s o f t h e C O D b a l a n c e f o r S y s t e m 6 a a re s h o w n i n
T a b l e A 2 .
Nitrogen balance calculat ions. R e f e r r i n g t o e q u a t i o n
( A 1 3 ) , t h e m a s s o f N O 3 - N i n t h e e f f l u e n t i s:
M N ,N = 2 6 3 m g N / d
F r o m e q u a t i o n ( A 1 4 ) , t h e m a s s o f T K N i n t h e e f f lu e n t i s:
MN.Te = 100 m g N /d
Table A2. COD balance calculations for
Sys tem 6a [data f rom Wentzel et al. (1990)]
I n p u t C O D O u t p u t C O D
(mg C OD /d) ( rag C OD /d)
QSTi 12 , 750 MC OD. ~ 1000
MooD .. . 3649
Mcom,~i 3899
Total 12,750 Total 854 8
% COD balance = 67.0%
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Table A3. N itrogen balance calculations for
Sys tem 6a [data f rom Wentzel
e t a l
(1990)]
I n p u t N O u t p u t N
(mg N/d) (mg N/d)
0 H ,
1050 Mrqn~ 263
MN.zc 100
MN .. . 247
Mde.i,.T 415
Tota l 1050 To ta l 1024
N balance = 97.5
T h e m a s s o f N l e a v i n g w i t h t h e w a s t e s l u d g e i s g i v e n b y
e q u a t i o n ( A I 5 ):
M N . .. = 2 4 7 m g N / d
T h e t o t a l o u t p u t N i s t h e n c a l c u la t e d b y s u m m i n g th e a b o v e
v a l u e s , t o g e t h e r w i t h M d e .i t,T c a l c u l a t e d e a r l i e r [ i.e . e q u a t i o n
( A l 6 ) ] , g i v i n g :
o u t p u t N = 1 0 2 4 m g N / d
E q u a t i o n ( A l 7 ) i s u s e d t o c a l c u l a t e t h e t o t a l i n p u t N :
i n p u t N = 1 0 50 m g N / d
T h e r e f o r e t h e N b a l a n c e i s g i v e n b y :
N b a l a n c e = ( o u t p u t N / i n p u t N ) * I 0 0
= ( 1 0 2 4 / 1 0 5 0 ) * 1 0 0
= 9 7 . 5
T h e r e s u l t s o f t h e n i t r o g e n b a l a n c e f o r S y s t e m 6 a a r e s h o w n
i n T a b l e A 3 .
WR
2 9 / 2 ~ Q
