Main considerations for a DEWATS design include:
• the substrate’s property (water consumption, feedstock, chemical oxygen demand (COD) and volatile solids (VS));
• operation data (temperature, organic load rate (OLR), hydraulic retention time (HRT) and sludge retention time (SRT));
• performance expectation (biogas (methane (CH4)) production, COD removal);
• post treatment (digestate property and sludge disposal or reuse); and
• nutrient recovery methods.
The laboratory (Fig 1) is equipped with several instruments to assess the performance of physical and biological processes within a DEWATS. The important parameters which may be managed by the TED laboratory are COD, temperature, pH-value and sludge parameters.
Why are these parameters useful in evaluating the performance of a DEWATS?
• COD is a composite parameter and represents the pollution strength of wastewater hence it is an important indicator in assessing the treatment efficiency of any wastewater treatment system.
From domestic wastewater the conversion of 1 kg COD produces 0.35m3 CH4 (1m3 CH4 equals to 2.86 kg COD converted ). Biogas contains as main component 55 – 85 vol% CH4 and 15 - 45 vol% CO2. Therefore biogas is a direct indication for COD degradation. COD is also needed to evaluate the possible overloading of the biogas digesters as the OLR for unstirred anaerobic systems may not exceed 2kg COD/m3 digestion volume, while for anaerobic baffled reactors and anaerobic filter the maximum is 15 kg COD/m³/d.
To ensure homogenisation the samples are filtered before the analysis with a white ribbon filter, retention bigger than 4, smaller than 12µm. Standardized cuvette-tests are used for the analysis. Depending on the range up to 2ml sample has to be pipetted into the cuvette. The cuvette contains potassium dichromate which oxidizes nearly all organic oxidizable substances. The consumption of potassium dichromate is converted into an equivalent oxygen demand (the amount of oxygen which will be consumed if the oxidation has taken place using oxygen). The closed oxidizing phase takes two hours at 148°C.
After cooling down the adsorption of the cuvette is measured at a specific wavelength in a calibrated photometer where the yellow coloration of reduced potassium dichromate is evaluated and given out as oxygen consumption.
• All enzymatic processes depend directly on temperature. Generally these processes are more likely to take place with increasing temperature until the optimum level is reached. Monitoring of the temperature might be important especially in Lesotho where air temperatures can drop far below zero in winter. Anaerobic processes are particularly sensitive to low temperatures and day/night temperature variations of more than 3°C. Psychrophilic anaerobic treatment is an attractive option for wastewater discharged at moderate to low temperatures (optimal temperature for psychrophilic microorganisms is around 17°C). It can be assumed that low temperatures in winter have a significant impact on both the treatment efficiency as well as the by-product biogas.
• Most bacteria only survive within small boundaries related to the pH-value. Different degradation phases have different optimum pH values. The pH-value might change significantly especially for DEWATS-plants where, in addition to the toilet blackwater, manual feeding with kitchen waste or animal dung is also done (co-digestion). The processes in the chain of biogas generation are the hydrolysis, where the microbial process is turning insoluble organic material such as fats, starches and proteins into soluble by-products; followed by acetogenesis where acetate from soluble organic materials (i.e. products of hydrolysis) is produced; and acidogenesis where organic acids from soluble and insoluble organic material are produced. High loads of batch feeding of carbon or nitrogen rich waste can cause a drop of pH and lead to an inhabitation of the methane generating bacteria as the last step in the chain of biogas production. The optimal pH boundaries for methane producing bacteria are between 6.5 and 8.1, according to the optimum of the methanogenesis. If methane is not produced by overfeeding the treatment efficiency will decrease according to COD removal.
• The sludge bed thickness is an indicator for the developed biomass in the anaerobic baffled reactor (ABR). It increases with the time of operation as no biomass is flushed out. In general a correlation between sludge level and COD removal exists, meaning that a certain sludge level is required to ensure a stable removal of COD.
The laboratory is fitted with professional pipettes, vessels and other accessories to ensure high quality analysis with low measurement errors during the analysis. In addition, the storage of the samples is very important after sampling (Fig 2) and before analyzing. For these reasons the samples are stored in a cooling box at 4°C to avoid biological processes taking place after the sampling is done.
Fig 2: taking samples from the second treatment module ABR Fig 3: sampling locations
Indicative first results:
To look into the behaviour of the performance over a course of one year samples were taken in April, May and June from two sites (Figure 3 shows a flow chart including the locations of sampling). Site A (Children´s Home) has been in operation since 2008 while site B (St. Angela) was commissioned in 2009. Both systems have been designed for a very similar daily hydraulic load therefore the size of the treatment modules differs only slightly. The wastewater sources are toilet, bathroom, kitchen and laundry as well as solid kitchen waste. Site B, in addition, is fed twice a week with manure from up to 600 chickens. All wastewater sources (as well as the additional feeding for site B) enter the DEWATS-plant at the digester. The sludge bed thickness in the second treatment module ABR (first chamber) was assessed with 22cm for site A and 21cm for site B. The plants at the post-treatment step planted gravel filter (PGF) cover up to 75% on site A and less than 10% on site B.
The conclusion from the effluent quality is that the performance of both sites is unsatisfactory. After the post treatment (PGF) 319mgCOD/l was measured for Site A and 367mgCOD/l for site B. The treatment efficiency of the second and third treatment steps related to COD was found to be, on average, 52% and 65% respectively. In earlier studies conducted in 2008 site A achieved COD effluents of 350mgCOD/l and COD reduction of 66%.
Overall, the older system (site A) achieves slightly better effluent quality, therefore, the COD removal efficiency of the younger system (site B) is a little better. According to an earlier study the performance of site B improved to only a minor extent. However, both systems reach the South African irrigation standard of 400mgCOD/l for small scale systems with not more than 500 m³/d discharge. A further interpretation of the above observations is difficult as the HRT and VS of the two systems could not be assessed within the first attempt. An extended monitoring program is therefore planned for which additional measuring devices, such as water meters and gas meters, will be necessary.
The influence of temperature during winter, mentioned earlier, seems not to affect the treatment performance of Site A. Figure 3 shows the COD concentration and temperature plotted against the timeline. The temperature dropped from 23°C starting in April to 12°C end of June. In the same time period the influent concentration at the second treatment step, ABR, increased from 720 to 1143mgCOD/l whereas the effluent concentration fell from 519 to 260mgCOD/l.
Fig 4: COD concentrations, Influent and Effluent, Site A
These interesting observations are contrary to the assumption made above. The plant seems to improve its performance related to COD removal at low temperatures and additional higher influent concentrations. It is possible that the influent concentrations have been relatively high for a couple of weeks with a low hydraulic load. This causes a higher HRT and OLR and could have led to an accession of biomass, especially of psychrophilic microorganisms, as their optimum temperature is around 17°C. There is a cold domain from 0 to 17°C in which the temperature characteristic is twice as high as the suboptimal domain from 17 to 30°C.
C. GUILLOU AND J.F. GUESPIN-MICHEL, Evidence for two domains of growth temperature for the psychrotrophic bacterium Pseudomonas fluorescens 1996. Appl. Environ. Microbiol. 62(9):3319-3324. aem.asm.org/cgi/reprint/62/9/3319.pdf
WIKIPEDIA: HTTP://WWW.WIKIPEDIA.ORG; The free encyclopedia (24/06/2010)
WASSER-WISSEN: HTTP://WWW.WASSER-WISSEN.DE/ABWASSERLEXIKON; Institut fuer Umweltverfahrenstechnik, Universitaet Bremen (24/06/2010)
FOXON K.M., PILLAY S., LALBAHADUR T., RODDA N., HOLDE F. AND BUCKLEY C.A.: The anaerobic baffled reactor (ABR): An appropriate technology for on-site sanitation, Pollution Research Group, School of Chemical Engineering, University of KwaZulu-Natal, Durban 4041, South Africa and Biochemical Research Group, School of Life and Environmental Sciences, University of KwaZulu-Natal, South Africa and Centre for Water and Wastewater Research, Durban Institute of Technology, South Africa 2004 Water Institute of South Africa (WISA) Biennial Conference, Cape Town, South Africa, 2004 www.bvsde.paho.org/bvsacd/cd27/reactor.pdf
MUELLER C.: Decentralized Co-Digestion of Faecal Sludge with Organic Solid Waste, Case Study in Maseru, Lesotho Dübendorf, 2009 (unpublished)