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MINERALISATION AND PATHOGENREMOVAL IN GRAVEL BEDHYDROPONIC CONSTRUCTEDWETLANDS FOR WASTEWATERTREATMENT
J. Williams*, M. Bahgat**, E. May***, M. Ford*** andJ. Butler*
* School of Civil Engineering, University ofPortsmouth , Burnaby Road. PortsmouthPO] 3QL, UK** Faculty of Botany. Suez Canal University. Ismailia , Egypt*** School ofBiological Sciences. University of Portsmouth . King Henry] St. ,Portsmoutlt POI 2DY. UK
ABSTRACT
Gra vel Bed Hydroponics (GBH) is a construc ted wetland system for sewage treatment which has provedeffective for tertiary treatment in the UK and secondary treatment in Egypt. Significant impro vements ineffluent quali ty have been observed in 100 m long field scale heds planted with Phragmit es australis,res ulting in large reductions in BOD. suspended solids and amm oniacal N. For such GBH beds. operaungoptimal ly with a residence time of about 6 hours . 2 to 3 log cycle reductions in the counts of indicatorbacteria , certain bacterial pathogens and viruses are typicall y obtai ned . However, the efficiency ofmineralisation was s trongly influenced by flow-rate and the prevailin g temperature. In addition, in the UK,overloading of the treatment system rednced the efficien cy of rem oval of faecal coliforms, probably due to
decreased adsorption to biofilms, Faecal coliform counts were also more strongly correlated to BOD thansuspended solids . As a secondary treatment process, pathogen removal was consi stentl y better in Egypt thanthe UK . Although GBH constructed wetlands do not fully satis fy the WHO guidelines for unrestrictedirrigation, they can make a significant contribution to the control of path ogens in de veloping countries.
Constructed wetlands are a sewage treatment technology which may be appropriate for some developingcountries. Such systems have low energy and maintenance requirements. Operating skills tend to be similarto agriculture and irrigati on. unlike the technically demanding control and maintenance requirements ofconventional sewage treatment plants which can pose problems in many locations. In developing countriesthere are concerns about the public health implications of relatively unrestricted effluent disposal and reuse.
49
50 J. WILLIAMS et al.
Therefore, the main objective in wastewater treatment is often the removal of potentially pathogenicmicroorganisms (WHO, 1989).
GBH engineered ecosystems have been applied to treatment of industrial and domestic wastewaters in theUK and Egypt. Field scale GBH systems have been operated for five years and an extensive database ofphysical and chemical treatment information has been established. Such systems consist of gravel filledinclined trenches which are planted with reeds (predominantly Phragmites australis). Horizontal subsurfaceflow of sewage comes into contact with biofilms on the surface of gravel and reed roots where microbialactivity removes noxious sewage components. Exudation of oxygen and soluble carbon compounds from thesurface of roots stimulates nitrogen cycling within the reed rhizosphere (Williams et al., 1994). It has beenproposed that pathogen removal within gravel filled wetlands occurs by a variety of mechanisms;adsorption, sedimentation, filtration, predation and inactivation due to environmental stress are all thought tohave a role (Gersberg et al .• 1989). This paper examines mineralisation and the removal of bacteria and virustracer organisms during sewage treatment in Gravel Bed Hydroponic (GBH) constructed wetland systems .
MATERIALS AND METHODS
Details of the design, construction and operation of GBH systems have been presented elsewhere (Butler,1991; Butler and Dewedar, 1991; Butler et al.• 1990; May et al., 1990). This paper will compare treatmentbetween 100 m long beds planted with Phragmites australis performing tertiary treatment in the UK andsecondary treatment in the UK and Egypt. All the beds were lined with an impermeable geomembrane laidon a sand base. Sewage was applied via a 'v' notch weir. Drilled tubes were set in the gravel to monitorconditions in the beds. Residence times in the systems were approximately 6 hours.
The UK beds at Budds Farm STW. Havant (Southern Water Services Plc) were 1.4 m wide. 0.2 m deep witha gradient of 1:100. The beds were filled with flint gravel 10 to 20 mm in diameter and operated in anintermittent manner. One bed was fed with primary settled sewage and one with secondary treated effluent.applied for 12 hours a day. Hydraulic loadings for the system have been calculated by dividing the total flowentering the beds per day via the 'v' notch weir by the bed surface area. The tertiary bed was loaded at amean of 7.9 mmld and the secondary bed at 5.8 mmld.
Three 100 m long beds at Abu Attwa STW (Ismailia. Egypt) were 2 m wide, 0.3 m deep and had a gradientof I:20 over the first 3 m; 1:40 between 3 and 13 m; I:50 between 13 and 23 m; and I:100 over the rest ofthe bed. They were operated for 15 hours a day at a mean loading (calculated as above) of 11 mm/d. Onebed was filled with flint gravel, one with limestone and one with dolerite of approximately 10 to 20 mmdiameter. The results from these three beds have been pooled to create a reliable data set.
Biochemical oxygen demand (BOD) and suspended solids were measured by standard techniques (APHA,1992). Ammoniacal nitrogen was measured colorimetrically using the alkaline phenate method and totaloxidised nitrogen (TON) was determined by reduction of nitrate to nitrite and subsequent colorimetricquantification of nitrite by the azo-dye method. Dissolved oxygen (DO) was measured using an ElL 8012oxygen probe.
Total heterotrophic bacteria were counted by pour plating in nutrient agar. followed by incubation at 22°Cfor 24 hours. Presumptive counts of total and faecal coliform bacteria were obtained by standard plate countprocedures (APHA. 1992). Presumptive Salmonella sp. were counted by spread plating onto xylose-lysinedesoxycholate (XLD) agar (Difco) and counting black-centred red colonies visible after 24 hours incubationat 37°C. Presumptive Vibrio cholerae were counted by plating onto TCBS agar (Oxoid), yellow coloniesvisible after 24 hours incubation at 35°C were counted (APHA, 1992).
Bacteriophage have been proposed as useful tracer models for the fate of enteric viruses during sewagetreatment. They tend to occur in higher numbers in sewage. and are technically much easier to count thanpathogenic viruses (IAWPRC, 1991; Qureshi and Qureshi. 1990). Coliphage capable of lysing E. coli B(NeIMB) were counted by the agar overlay method (Adams. 1959). Samples were filtered (0.45 um),
Gravel Bed Hydroponics 51
diluted and added to half strength nutrient agar seeded with E. coli culture. Plaques greater than 2 mmdiameter were counted in the bacterial lawn and results expressed as plaque forming units (pfu) per ml.Culturable enterovirus counts were performed in the laboratories of Severn-Trent Water Pic. Plaque countswere made in tissue culture using an African Green Monkey Kidney cell line (Morris and Waite, 1980).
RESULTS
Chemical and physical perfonnance characteristics
The results from Budds Farm were collected between Autumn 1988 and Autumn 1991 when monthlymonitoring of physical and chemical parameters was undertaken. The Abu Attwa chemical data is based onmonitoring carried out between Winter 1988 and Winter 1990. Chemical data are means based on more than20 sampling occasions and analyses were done on the same water samples to create a multivariate data setwhere characteristics could be compared directly.
Table I shows the mean inlet and outlet concentrations of BOD, suspended solids, ammoniacal N and totaloxidised nitrogen (TON). These have been used to estimate percentage treatment efficiencies for eachsystem. The inlet concentrations show the typical effluent quality entering the GBH systems. The tertiaryGBH bed received a well nitrified effluent with a relatively low BOD and effectively polished the effluent,giving notably lower concentrations of BOD and ammoniacal N in the effluent. The suspended solidsdetected in the effluent were composed of a variety of humic material and reed detritus, thus furtherbiological treatment would have little impact on this recalcitrant material. Secondary treatment beds in theUK received a stronger feedwater than the secondary treatment beds in Egypt. The UK secondary bedeffected large reductions in BOD and suspended solids, but had little impact on ammoniacal Nconcentrations. However in Egypt, large reductions in suspended solids and BOD were accompanied by asubstantial decrease in ammoniacal N concentrations.
Table I. Changes in BOD, suspended solids, arnmoniacal-N and total oxidised-N in 100 m GBH bedsplanted with Phragmites australis
Figure 1. Mean concentration profiles of biochemical oxygen demand (BOD), ammoniacal nitrogen (Amm N) anddissolved oxygen in the 100m long GBH beds planted with Phragmites australis over a two year period.
Gravel Bed Hydroponics 53
Concentration profiles of BOD, ammoniacal N and dissolved oxygen are plotted against distance from theinlet for the three systems in Fig. 1. As can be seen from Fig. Ia, there was an increase in BOD over theinitial section of the GBH beds at Abu Attwa reaching more than 170 mg/1. This was due to filtration andsedimentation of organic solids from the sewage in the gravel matrix. After a few months, this accumulationof material reached a steady state, indicating that the rate of accumulation was balanced by the rate ofdegradation. Over the next 30 m of bed, BOD decreased to less than 50 mg/l and further reductions werethen seen over the remaining 55 m of bed length. This accumulation of degradable organic matter did notoverload the system since dissolved oxygen concentrations increased steadily down the bed and ammoniacalN concentrations fell with distance. Figure Ib shows the profiles during secondary treatment in the UK.Accumulations of organic matter increased BOD to more than 300 mg/l over the first 20 m of the bed,although this fell to less than 100 mg/I by 40 m. In this case the degradation of accumulated material didoverload the reaeration capacity of the bed as dissolved oxygen concentrations did not increase until 50 mand only reached about I mg/l by the end of the bed. These relatively anoxic conditions meant that there wasno significant nitrification of ammoniacal N to nitrate in the system. The tertiary treatment bed in the UKalso had an accumulation of organic matter in the first 20 m giving a maximum BOD of about 55 mg/l,however, as Fig. Ic shows, the mean dissolved oxygen concentration remained in excess of I mg/1. Whilebeyond 20 m. when BOD fell very quickly, highly aerobic conditions were soon established. Theammoniacal N profiles showed an initial increase over the first 20 m (to more than 6 rng/l) followed by asteady reduction to less than 0.5 mg/l over the remainder of the bed.
The influence of temperature and hydraulic loadin~ on treatment
The multivariate monitoring regime included measurements of both operational and environmentalconditions at the time of sampling and it was thus possible to assess the effect of climate and flow rate on theperformance of the system. Carbon mineralisation, measured as BOD and suspended solids, was examinedover selected temperature ranges. In other experiments flow rate was adjusted over defined ranges in orderto describe the effects on nitrogen mineralisation measured in terms of ammoniacal nitrogen and totaloxidised nitrogen.
BOD and SS (m g/I)50
• BOD40
55
30
20
10
o12-17 17 -22 22-27 27-32
Temperature Range (degrees C)
Figure 2. The effects of sewage temperature on biochemical oxygen demand (BOD) and suspended solids (SS) inthe effluent of the 100 m Phragmites beds in Egypt.
Figure 2 illustrates the influence of sewage temperature on BOD and suspended solids in treated effluentfrom the Egyptian GBH system. The mean over each temperature range is estimated from between 5 and 18samples and the standard error of the mean (SEM) is shown by error bars. Below 17°C, BOD was typicallygreater than 25 mg/I, while at temperatures in excess of 27°C it was rarely more than 15 mg/I. The
54 J. WILLIAMSet al.
relationship between suspended solids and temperature was more pronounced, with mean concentrationsvarying from 40 mg/l to 10 mgll over the same temperature range.
Figure 3 shows the mean concentrations of mineral nitrogen in Abu Attwa GBH effluents over flow rateranges. The mean for each flow rate range is based on between 4 and 12 sampling occasions and the SEM ofthis mean is shown by the error bars. Effluent ammoniacal N concentrations were lowest at very low flowrates but the most discernible effect was a large increase in mean ammoniacal N concentration from 7 tomore than 18 mg/I at flow rates in excess of 13.5 mm/d.
Amm N and TON (mg-N /I)20
Figure 3. The effects of flow rate on ammoniacal nitrogen (Amm N) and total oxidiscd nitrogen (TON) in theeffluent from the 100 m Phragmites beds in Egypt.
, I
40 60 80 100 120 140
Distance (m)
b) Secondary Treatment
log cfu/ml6,-----------------,
5 ! i ~I' •
4
3
2
1
0 0 20o 10 20 30 40 50 60 70 80 90 100
Distance (m)
a) Tertiary Treatment
log cfu/ml6'---~~~~~~~-~~~~---'
5
4 I •I
3
2
1
OL-------------'
Figure 4. Profiles of faecal colifonns in 100m long Phragmites GBH treatment beds in the UK. The secondary bedincludes a 40 m crop bed.
Microbiolol:ical treatment
The bed profiles of total bacteria and coliforms in the UK were performed on more than 8 samplingoccasions between April 1989 and August 1991. Coliphage profiles were determined on 3 occasions in theSummer of 1991; enteroviruses were also enumerated during this period on triplicate samples taken from 5positions in the beds. The Egyptian microbiological data are based on more than 20 sampling occasionsbetween January 1992 and March 1993 and counts of microorganisms were estimated in samples of watertaken from sampling tubes set in the beds. Figure 4 shows the scatter plot profiles of faecal coliform countsin the UK GBH beds. Counts are plotted against distance from the inlet. In general, exponential reductions
Gravel Bed Hydroponics 55
with distance were seen in each system, although large variations were observed between samplingoccasions. The large variation at 100 m in the secondary system may be due to a discontinuity in the bed atthis point where a connecting pipe to the subsequent crop bed may have allowed sedimentation or exposureto sunlight.
In order to compare the removal rates of bacteria and viruses which are indicative of faecal contaminationwithin the different GBH systems, it was found appropriate to estimate decimal reduction distances (DRD).The DRD is the length of bed required to give a ten fold reduction in counts of particular microorganisms.The gradients of linear regression lines describing significant negative correlations between log 10microbialcounts and distance from the inlet (Equation i) were used to estimate DRD (Equation ii). All the linearregression models were p<0.05.
i) Linear regression equation;Microbial Count 10gIO =a + b [Distance from Inlet (m)]where a =intercept;
b =gradient (11m)ii) Decimal Reduction Distance
DRD (m) =-lib
Table 2 Removal of potentially pathogenic organisms during passage through 100 m long GBH beds plantedwith Phragmites australis
*=none detected :oIa= not applicable due to low sample numbers: nd = not determined.
56 J. WILLIAMS et al.
The reductions in numbers of indicator organisms, illustrated by the DRD values in Table 2, indicate that allthe GBH beds had the capacity to remove potentially pathogenic microrganisms. DRD values for a range ofbacterial groups during tertiary treatment were between about 40 and 50 m. Slightly better removals wereseen during secondary treatment in Egypt, suggesting that 100 m long GBH beds can consistently give 2 to 3log unit reductions in bacterial counts. However, lower removals of 1 to 2 log units were seen for totalbacteria and faecal coliforms in the UK secondary treatment bed. Coliphage counts also suggested that 2 to 3log unit reductions in indicator viruses were occurring in all the beds, this was supported by enteroviruscounts in the UK when >2 log unit reductions were observed.
DISCUSSION
Removal of solid particles and consequent stabilisation of organic matter (evidenced by reductions in BODand suspended solids) was seen in all the GBH systems. However, the effective net oxidation of ammoniacalN seen during tertiary treatment and secondary treatment in Egypt. was not found during secondarytreatment in the UK (Table I). This may have been due to the relatively anoxic conditions over most of thebed length (Fig. lb) inhibiting the activity of the obligate aerobic bacteria responsible for the nitrificationprocess. These conditions occurred when the oxygen demand of organic matter entering the systemexceeded the oxygenation capacity of the beds. Anaerobic degradation processes are relatively slow at theambient temperatures of the British Winter and organic matter thus accumulates in the system, blockingpores in the gravel matrix leading to overland surface flow of sewage short circuiting the treatment zone.This was manifested during secondary treatment in the UK, when anoxic conditions and overland flowextended to more than 50m of bed length (Fig. Ib). In the knowledge of this organic matter accumulation,the Abu Attwa beds were built with steeper gradients near the inlet. This modification, combined with themore favourable climate, meant that this problem was rarely encountered in Egypt. In addition, stabilisationand mineralisation of organic matter was more efficient at higher sewage temperatures, and this was clearlyapparent in the observed reductions in mean effluent BOD and suspended solids concentrations (Fig. 2).
The GBH beds at Abu Attwa were operated at a mean hydraulic loading of 1 m2 per head of population(assuming water usage of 100 l/day per head). Investigations into varying flow rates suggest that thehydraulic loading to beds should not exceed 13.5 mm/d, this is equivalent to 0.66 m2 per head of population.At higher flow rates, treatment decreased, shown by increases in ammoniacal N concentration in theeffluents (Fig. 3). Reductions in nitrification are often the first indication of overloading, since the slowgrowing autotrophic nitrifiers are particularly sensitive to oxygen depletion. Similar increases in ammoniacalN were seen at high flow rates in the UK beds (Williams, 1993).
Clearly, serious overloading can affect the treatment efficiency of GBH beds with respect to themineralisation of organic matter and nitrogen transformations. However, there is also evidence that pathogenremoval is affected by operational procedures. During tertiary treatment in the UK, faecal coliform countshad a stronger correlation with BOD than suspended solids:
Faecal Coliforms (log 10 cfu/ml) = 0.074 x BOD + 1.95
where n =40, r =0.59 and p =<0.01.
This association with degradable organic matter suggests that adsorption, the predominant removalmechanism of BOD in biofilm systems (Gray, 1989), was also an important pathogen removal process,probably more important than sedimentation. This has also been suggested for other gravel filled constructedwetlands (Gersberg et al., 1989). It is possible that the improvements in BOD treatment with increasingtemperature in Egypt (Fig. 2) are also likely to enhance pathogen removal. The lower removal of faecalcoliforms during secondary treatment in the UK (Table 2) could either be due to overland flow reducingcontact with these adsorption sites or anaerobic conditions which are reported to prolong faecal coliformsurvival in natural waters (Gray, 1989).
Gravel Bed Hydroponics 57
GBH beds can reduce the numbers of enteric bacteria and viruses in sewage. However, these reductionsalone may not be sufficient to satisfy the relevant water quality standards for bathing or reuse. The removalof bacterial and viral tracers in correctly loaded 100 m long GBH beds tended to be around 2 to 3 log unitreductions (Table 2). However, this may still be insufficient for GBH effluents to meet WHO guide-lines forunrestricted irrigation (WHO, 1989). Additionally, although preliminary investigations into the removal ofhelminth ova have shown promising results, further work is required and GBH systems can only be currentlyrecommended for WHO category C irrigation (making the effluent suitable for cereals, industrial and foddercrops). However, GBH systems can still offer significant pathogen removal and this may be a usefulcontribution to public health strategies controlling disease in developing countries. For such applicationsGBH beds may have potential as part of a modular system, incorporating other ecological systems such asponds or lagoons. In Europe, EC directives on eft1uent quality have addressed concerns about microbialcontamination of recreational and sensitive waters. Cheap tertiary treatment plants may be required at manysmall sewage treatment plants where the capital and operational costs of disinfection systems would beuneconomic. The EC Bathing Water Directive requires 100 cfuJml total coliforms and 20 cfu/ml faecalcoliforms to be obtained on a 95 percentile basis at bathing beaches (Gray, 1989). The mean values fortertiary treated eft1uent in Table 2 represent 50 percentiles and are around 100 cfuJml for both coliformgroups. Dilution of this eft1uent in sea water (with subsequent further inactivation of bacteria) mean it islikely that the EC standard could be achieved at locations where GBH tertiary treatment beds are installed.The land area requirements of tertiary treatment GBH beds appear to compare favourably with other lowcost alternative wastewater treatment technologies which have shown similar potential to remove pathogenicmicro-organisms, such as lagoons and soil percolation systems (Williams, 1993).
Therefore the envisaged applications of GBH sewage treatment systems are either for tertiary treatment,small household scale systems or for treating wastewater from small settlements where land is readilyavailable. The main constraint on their use in developing countries is the same as for all centralisedtreatment facilities, the expense of constructing the sewerage system. However, the development of smallbore sewers, such as that planned for a 4200 pe GBH treatment system in Takaddom village in Sinai, mayoffer a more practicable solution to these problems.
ACKNOWLEDGEMENTS
The experimental beds were constructed and operated with grants from the ODA and SERC made to lEB.JBW was sponsored by Southern Water Services PIc. The authors would like to acknowledge R. F.Loveridge and C. Hughes for help in the chemical sampling and analysis. We would also like to thank Prof.A. Dewedar and other colleagues from the Suez Canal University for their work in Egypt and the UK.
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