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Technologies for Economic Development

Water quality assessment


by Ruben Zahlten, TED

Fig 1: COD analysis

Ruben Zahlten TED has established a new small scale laboratory to conduct wastewater analysis for certain parameters. The laboratory was needed so measurement campaigns at different Decentralized Wastewater Treatment System (DEWATS) could be conducted to evaluate the treatment efficiency and effluent quality over the course of the year.

 

Main considerations for a DEWATS design include:

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?

Fig 2: taking samples from the second treatment module ABR
second
treatment module ABR
Fig 3: sampling locations
sampling locations
Fig 4: COD concentrations, Influent and Effluent, Site A
COD concentrations
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.

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 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.
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.

Literature:

Guillou, C. and Guespin-Michel, J.F. (1996). Evidence for two domains of growth temperature for the psychrotrophic bacterium Pseudomonas fluorescens MF0. Appl. Environ. Microbiol, 62(9):3319-3324.     Download (226 kB)

Wikipedia: http://www.wikipedia.org (24/06/2010)

Wasser-Wissen: http://www.wasser-wissen.de/abwasserlexikon (24/06/2010); Institut fuer Umweltverfahrenstechnik, Universitaet Bremen

Foxon K.M., Pillays, S., Lalbahadur, T., Rodda, N., Holde, F. AND Buckley, C.A. (2004).The anaerobic baffled reactor (ABR): An appropriate technology for on-site sanitation. Water SA, 30(5).     Download (506 kB)

Mueller, C. (2009). Decentralized Co-Digestion of Faecal Sludge with Organic Solid Waste: Case Study in Maseru, Lesotho. Technical report, EAWAG/Sandec, Dübendorf.     Download (6 MB)

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