Kuliah 3 Waste Stabilization Ponds and Lagoons

Waste Stabilization Ponds and Lagoons

MATA KULIAH SATUAN PROSES

DISIAPKAN OLEH : Dr. DAVID ANDRIO, M.Sc

1. CONCEPTS AND PHYSICAL BEHAVIORSTABILIZATION PONDS OR LAGOONS

Stabilization ponds or lagoons : the most common industrial wastewater treatment facility.

This versatile installation serves many basic purposes, including:

a. storage or impoundment of wastewater;

b. settling and removal of suspended solids;

c. storage or impoundment of settled solids;

d. equalization;

e. aeration;

f. biological treatment; and

g. evaporation


The main advantages and disadvantages of stabilization pond are listed below (1) :

A. Advantages : 1. Low operational and maintenance cost,2. Lagoons provide effective treatment with minimal threat to the environment3. Work well in clay soils where conventional subsurface on-site absorption fields will not

work.

B. Disadvantages

1. Lagoons must be constructed in clay soil or be lined to prevent leakage

2. May overflow occasionally during extended periods of heavy rainfall

3. If there are extended periods of overcast windless days, offensive odors may occur for a brief time

4. Can not be installed on a small lot. Takes up a relatively large space

5. Lagoons are not aesthetically acceptable to some people. Some people consider lagoons unsightly and unsafe.

6. As with any other open body of water, there is some potential danger. Although lagoons are required to be fenced, this does not always prevent access by people or pets.

(2) treatment systems in the general category of “stabilization ponds” usually serve small communities of whom 90% have populations of 10,000 persons or fewer (US EPA SURVEY)


1.1. Pond Ecology and Process Reactions

complicated reactions – both chemically and biologically – often occur in a pond.



Major processes can be identified in active stabilization ponds:

1. Sedimentation

2. aerobic decomposition

3. anaerobic fermentation

4. bacterial – algal symbiosis

5. oxygen transfer across the water surface

6. Sulfur bacteria actions

7. evaporation, and

8. seepage



1. Sedimentation

Precipitation : suspended matter from incoming wastewater and fecal matter from worms and insects. Precipitation can be enhanced significantly by chemical and biological flocculation in the pond.

Vigorous photosynthetic actions by algal cells result in a rise of pH, promoting the formation of calcium and magnesium flocs in the alkaline condition.

From 80 to 90% of suspended matter can be precipitated in a few hours, depending on the temperature, hydraulic flow regime, and depth of a pond.

The bioflocculation of synthesized bacterial cells and algal cells also constitute a part of the sedimentation regime.



2. aerobic decomposition

At DO at 0.1 to 0.2 mg/L, aerobic oxidation of biodegradable organics will take place in the pond similar to that in an activated sludge process.

Nitrification can take place in an activated sludge process, because ammonia is readily used by algae for growth.

At high pH, nitrogen stripping or precipitation as MgNH4PO4 could also take place before nitrification can become established.



3. anaerobic fermentation

Anaerobic decomposition of organic matter takes place in an established anaerobic zone in a pond. Both acid and alkaline fermentation occur concurrently to yield gases H2S, CH4, H2, N2, etc. that may escape the pond.

Odor production is often accompanied by a vigorous fermentation at high temperature and in the presence of high organic loading rates.



4. bacterial – algal symbiosis

Optimal condition for photosynthetic algae growth abundance much more O2 aerobic processes

Symbiosis occurs : turbidity of pond water is low and sunlight is plentiful. Photosynthetic action in the upper zone of the pond, algae grow in abundance, could supersaturate the zone with molecular oxygen. Abundant of oxygen supports the active aerobic oxidation by bacteria that in turn yield the inorganic nutrients NH3, PO3−

4 , and CO2 in particular, to meet the demand in algal growth.

CO2 sources : CO2 as an end product of bacterial oxidation and fermentation, from the atmosphere and from the inorganic carbon species in the CO2 - HCO−

3 - CO2−3 system. A vigorous photosynthetic

activity may lead to rapid depletion of CO2 and a consequent rise of pH in the pond.

7.6CO2 + 17.7H2O + HN+4 C7H8.1O2.5N + 7.6 O2 + 15.2 H2O + H+

2.3 g of CO2 are required for every gram of algal cell synthesized and 1.67 g O2 production is 1.67 g. the bacterial oxidation process without any other source of supply.




O2 can escape the pond and more often than not

The rate of O2 transfer depends on the deficit (or surplus if oversaturation).The rate constant K2 can be expressed in the following form:

K2 = (−1/t) log (Cs − C)/(Cs − C0)

where t = time, Cs = saturation concentration,

C = concentration at any time, C0 = initial O2 concentration,

and K2 = rate constant.

When Cs > C > C0, K2 is a reaeration constant.

When Cs < C < C0, K2 is a deaeration constant (oxygen stripping).




It is difficult to evaluate the rate constant K, because it is a function of surface renewal and temperature.

Ekman has developed empirical formulations for wind-induced current velocity and depth of circulation as follows :

P = V0/W = 0.0127√sinϴ, D/W = 11.1√sinϴ

V0 = surface-current velocity caused by the wind

W = wind speed;

(V0/W) = is a term called proportional surface velocity of the water (with observed magnitudes of p lies between 1 × 10−2 and 5 × 10−2 a function of latitude:

D circulation depth or depth of frictional resistance.




Knowing both p and D, the following equations according to Fair can be used for estimation of the reaeration constant K2;

G2 = 93 × 10−3(pW)3/(μgF) K2 = 29G3/D

W = wind speed in miles/hr; F = the fetch in miles; μ = the absolute viscosity with both μ and g in ft-lb-s units; G = the mean temporal velocity gradient (s−1); D = the circulation depth in ft.

oxygen transfer coefficient KL as a function of wind speed as follows (Bank) :

KL = kUm



6. Sulfur bacteria actions

aerobic oxidation of sulfur compounds in wastewater produces sulfate, whereas anaerobic bacteria reduce sulfate to sulfide.

Desulfovibrio sp. whose optimal growth occurs at pH 7.0 and ORP (oxidation reduction potential) at −100 to −300 mV and not grow at potentials higher than +27 mV

Sulfides can be oxidized using either molecular oxygen or CO2 as the hydrogen acceptor. The oxidation of sulfide is carried out in two steps in the absence of oxygen: CO2 + 2H2S→(CH2O) + H2O + 2S

3CO2 + 2S + 5H2O→(CH2O) + 4H+ + 2SO2−4



7. Evaporation

Rates of evaporation from ponds vary with temperature, vapor pressures of the water and the air in contact with it, wind speed, barometric pressure, and the salt content of water

E = 0.497(1 − 1.32 × 10−2Pa)(1 + 0.268W)(V − v)

E = evaporation (in/d)

Pa = barometric pressure (inch of mercury)

W = wind speed (miles/hr)

V = vapor pressures (inch of mercury) at the water temperature

v = vapor pressures (inch of mercury) at the dewpoint temperature of the atmosphere



8. seepage

Can be substantial depending on : wastewater quality and soil characteristics The seepage flow should be stopped if the quality of a nearby ground water supply is threatened.

water losses in anaerobic ponds constructed on sandy soils :Hart : In ponds 7 ft deep, the infiltration rate ranged from 6.6 cm/d

at an early stage of the operation to 4.1 cm/d 2 years later.Davis : stabilization ponds treating dairy wastes, infiltration rate of

122 cm/d in the beginning. The rate dropped to 0.5 cm/d after 4 months of operation.

physical entrapment of organic particulates in the pores of soil followed by the growth of slime-forming microorganisms. Sodium adsorption ratio of the wastewater can influence the permeability of stabilization pond soils. The sodium adsorption ratio is defined by US Salinity Laboratory as follows:

SAR (sodium adsorption ratio) = [Na+]/{0.5[Ca2+] + 0.5[Mg2+]}0.5


1.2. Biology of Stabilization Ponds Depends on : the type of stabilization pond, the influent wastewater

characteristics, the time of the day, and the season of the year. Because of limited control in operation, variations of biological population in stabilization ponds are much more apparent than in any other biological process.

1.2.1. Bacterial Population

stabilization ponds are operated at a very low bacterial population

Coli-aerogenes group is predominant in stabilization ponds. Achromobacter, Pseudomonas, and Flavobacterium are dominant in an aerobic pond, accounting for 90% of the total bacterial count.

Desulfovibrio sp. are abundant in stabilization ponds when substantial amounts of sulfate are present in wastewater. The Chlorobacteriacea and the purple Thirohodaceae then oxidize H2S to sulfur elements and sulfates in the presence of light.


1.2. Biology of Stabilization Ponds 1.2.2. Algal Population varies from one pond to another, 15 million/mL

Green algae usually dominate because they can adapt better to environmental changes such as extreme temperature and dissolved oxygen

In most ponds, coccoid green algae and green flagellates dominate the plankton throughout the year, while pennate diatoms and filamentous blue-green algae dominate the benthic flora

1.2.3. Zooplankton and Insects

Protozoa and rotifers can be found abundantly following significant increase of bacterial and algal population. help to stabilize the prey population and therefore the treatment performance.


1.3. Classification of Stabilization Ponds

According to the type of biological transformation and methods of oxygen supply:

1. Anaerobic Ponds

These are deep ponds where anoxic condition prevails throughout. Organic loadings are very high and BOD removal is limited to 80% or below. Further treatment of the anaerobic pond effluent by aerobic ponds is usually required.

2. Facultative Ponds

Facultative ponds receive medium to low organic loadings. Generally they are 8 ft deep or shallower. The bottom layer is usually anaerobic, but the surface layer is kept aerobic through photosynthesis and surface reaeration. BOD removal is higher than that of anaerobic ponds.

3. Aerated Lagoons

4. High-Rate Aerobic Ponds




3. Aerated Lagoons

Oxygen supply : relies almost completely on mechanical aeration devices. Either air diffusion or mechanical aeration can be used.

Depending on the power level used for aeration :

A. as aerobic lagoons : lagoons.With power levels at 0.03 hp/1000 gal (6W/m3) or above




3. Aerated Lagoons

B. aerobic-anaerobic lagoons :

lower power levels, an anoxic bottom layer can be expected;

equivalent to a facultative pond, except that the former uses mechanical aeration for oxygen supply;

usually are deeper than facultative ponds and receive medium to high organic loadings;

Treatment efficiency can be very high;

The mixing characteristic that separates aerobic (near complete mix) to aerobic- anaerobic (poorly mixed) is more dependent upon the pond geometric parameters than upon operating parameters such as horsepower input.




4. High-Rate Aerobic Ponds

These are shallow ponds of only 12 to 18 in. working depth.

Light penetration is essential to maximize algal production in which the bacterial-algal symbiosis affects BOD and nutrient removal.

Very high organic loading is allowable with good BOD removal. There are also stabilization ponds called tertiary ponds, multiple stage ponds, and integrated ponds.

2. SYSTEM VARIABLES AND CONTROLSTABILIZATION PONDS OR LAGOONS

The time lag depends on the degree of mixing in the pond and seasonal variation of temperature, low concentrations of biological solid

The sludge age is identical to the hydraulic detention time because no sludge recycle is practiced

A material balance around a pond taken as a complete-mix reactor will yield the following:

dS/dt (V) = QS0 − QS − kSV “ the rate change of substance concentration S in a pond of liquid

volume V is equal to the rate of substrate inflow (Q = flow rate, So = influent substrate concentration) minus the rate of substrate outflow (S = outflow substrate concentration, which is identical to the substrate concentration in the pond), minus the rate of substrate removal (k = substrate removal rate constant)”.

Steady state : S/So = 1/{1 + k(V/Q)} = 1/(1 + kt), t = hid. detention time

Si/Si−1 = 1/(1 + kiti) multiple-stage ponds in series results in



2.2. Oxygen Supply

Major sources of oxygen supply in practice are (a) mechanical aeration and (b) photosynthetic oxygenation.

Oxygen is also consumed in the presence of H2S gas emitted from zones of anaerobic decomposition in the pond. H2S + 2O2 H2SO4

The overall photosynthetic efficiency E then can be calculated as:

critical photosynthetic efficiency necessary to provide enough O2 to meet the BOD loading given by Oswald :


2.2. Oxygen Supply


2.3. Temperature Effect

kT = k20ϴ(T−20)

The value of the temperature coefficient :

2.3.2. Heat Loss and Heat Conservation

Heat loss consists of heat loss evaporation He, convection Hc, and radiation Hr

According to Barnhart : H = He + Hc + Hr

He = 0.00722 HvC(1 − 0.1W)(Vw − Va)Hc = (0.8 + 0.32 W/2)(Tw − Ta) (34) Hr = 0.1(Tw − Ta)

Hv is the latent heat of vaporization; C is a constant characteristic of the pond; W is mean wind velocity; Vw is vapor pressure at water surface, in. Hg; Va is vapor pressure in atmosphere, in. Hg; Tw is pond water temperature, ◦F; Ta is air temperature, ◦F.

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Kuliah 3 Waste Stabilization Ponds and Lagoons

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