Water quality assessment
by Ruben Zahlten, TED
Fig 1: COD analysis
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 substrate’s properties: 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
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.
Fig 2: taking samples from the second treatment module ABR
Fig 3: sampling locations
Fig 4: COD concentrations, Influent and Effluent, Site A
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)