Decentralised treatment of concentrated sewage at low temperature in a two-step anaerobic system: two upflow-hybrid septic tanks

Abstract

The decentralised treatment of concentrated sewage (about 3,600 mgCOD/l) at low temperature was investigated in a two-step anaerobic system: two-anaerobic hybrid (AH) septic tanks (each 0.575 m3). The two reactors were placed in a temperature controlled-room and the HRT was 2.5 days for each reactor. The system was fed with concentrated domestic sewage, mainly black water from about 40 toilets flushed with only 4 litre of water and a limited amount of grey water. The system showed high removal efficiency for the different COD fractions. Mean removal efficiencies in the two-step AH-septic tank at 5 days HRT and 13 degrees C were 94, 98, 74 and 78% for total COD, suspended COD, colloidal COD and dissolved COD respectively. The results of short run experiments indicated that the presence of reticulated polyurethane foam (RPF) media in the AH-septic tank improved the removal of suspended COD by 22%. The first AH-septic tank was full of sludge after 4 months of operation due to the high removal of particulate COD and the limited hydrolysis at low temperature conditions. Therefore, a simple mathematical model was developed based on ADM1 (the IWA model in 2002). Based on the experimental results and the mathematical model, only a one-step AH septic tank is required. An HRT of 5.5-7.5 days is needed for that one-step AH septic tank to treat concentrated sewage at a low temperature of 13 degrees C. Such a system can provide a total COD removal as high as 87% and will be full of sludge after a period of more than a year.

Evaluation of decentralized treatment of sewage employing Upflow Septic Tank/Baffled Reactor (USBR) in developing countries

“…[2][3][4] show that in the first month of steady-state operation the second compartment compensated for the low efficiency of the first compartment, where the removal of BOD, COD, and TSS took place in both compartments. After that, most of the BOD, COD, and TSS concentrations were removed by the first compartment "upflow anaerobic compartment."…”

“…Many anaerobic modified septic tank systems were used and tested at different countries [3,4,5,6]. Panswad and Komolmethee [3] used full-scale septic tank/anaerobic filter unit with the tank's retention time varying from 22.5 to 90 h. They recommended a rather high retention period of not less than 48 h if the Thai effluent standards are to be met.…”

“…Elmitwalli et al [4] used two-step anaerobic system to treat sewage. They tested the performance of the two upflow-hybrid septic tanks which require high power input or high excavation depth due to that the two treatment steps exist in a vertical order.…”

Mathematical Modelling of Anaerobic Reactors Treating Domestic Wastewater: Rational Criteria for Model Use

“…This model was expanded to two stage first order (Rodrigues et al 2004) to predict organic acid concentrations. A limited subset of the ADM1 was applied to a two-stage upflow septic system (Elmitwalli et al 2003). The two stages were hydrolysis, and biological growth (or uptake), with decay.…”

“…A simplified model structure (Elmitwalli et al 2003) (recommended for design), as well as that of the ADM1 (recommended for operation/technology development) is shown in Figure 2.…”