POST TREATMENT OF UASB EFFLUENT BY SEQUENCING BATCH REACTOR (SBR)

Abstract

In this study, a bench scale Sequencing Batch Reactor (SBR) was used to investigate this . simple add on method for the post treatment of UASB effluent to meet present and future receiving water quality. Effluent from two UASB treatment plants, the 111 MLD STP at Bhattian and the 152 MLD STP at Balloke (both in Ludhiana) provided feed for two separate investigations in the study. The SBR unit (27 L Top Water Level, 13.5 L Bottom Water level) was. operated under non limiting dissolved oxygen conditions in each investigation using .simple repetitive cycles of Fill-Aerate, Settle and Effluent Removal (Decant). Influent during Settle and Decant sequences was positively interrupted to provide a best settling environment and to insure there was no bypass or short circuiting of influent. The SBR was not operated for defined anaerobic, anoxic, oxic sequencing that is a feature of the full scale 105 MLD SBR STP at Balloke. • At the end of the program a short study was initiated at the Balloke STP in order to adapt the simple SBR to simulate operation for Simultaneous Nitrification Denitrification (SND). .None of .the studies were intended to find optimum SBR sequencing and cycling protocols; their objective was to provide screening tests in order to establish the efficacy of the • combined treatment technologies as a means for meeting present and future effluent quality receiving water discharge requirements. Worldwide .over 200 full-scale UASB reactors are functioning for the treatment of both domestic and industrial wastewaters. At present some 23 full-scale .UASB based STPs are in operation at.various places in India with total capacity of about 985 MLD (Tare et. al., 2003). According to MoEF (Ministry of Environment and Forest) (2005, 2006), about 30 UASB based STPswere installed in India since late 1980sand more than 20 were then underconstruction. Though the UASB technology is promoted as a cost efficient and a relatively cheap method for.the treatment of domestic wastewater it is unable to reduce BOD, COD and TSS to the desired permissible limits of current and likely future effluent standards for surface water discharge. Being an anaerobic process it is unable to remove nitrogen or phosphorus. The effluent biological oxygen demand (BOD) of most of theanaerobic treatment systems, including UASB reactors treating domestic sewagewithout any posttreatment, has been reported to vary from 60 to 150 mg/L(Chernicharo, 2006) with the COD andtotal suspended solids (TSS) ranging betweenl00 to 200 and 50 to 100 mg/L, respectively (Foresti et al., 2006). Total nitrogen and phosphorous content in UASB effluent can be expected to be similar to that contained in raw domestic sewage SBR activated sludge, a well adapted and accepted technology, was used for the post treatment option of this investigation. The. SBR was operated over four runs. In the first run the, SBR was operated without sludge wasting in order to note a possible fate of reactor efficiency at increasing MLSS concentrations. The second and third runs were maintained at MLSS concentrations between 2200-4500 mg/L. The second run was operated with nitrifying seed sludge from the 27 MLD. STP at Haridwar, the third run was operated with nitrifying seed sludge from the 3.5 .MLD STP at .Rishikesh and the fourth run was operated with sludge from 105 MLD C Tech based plant Balloke, Ludhiana. The effluent BOD, COD and TSS content in all runs was well below the recommended effluent standard for discharge to receiving waters. The presence of heavy metals and especially zinc in the Bhattian UASB effluent was shown to significantly inhibit the onset of -nitrification. Inhibition of nitrification was confirmed using - a Fed Batch Reactor (FBR) :protocol which was applicable- to any dispersed or attached growth methodology that is inhibited by a toxic input. Whilst initially considered to be an apparent problem it is easily solved by conventional chemical precipitation in order to remove the offending heavy metals • It is generally ,recognised by.the industry that combined UASB and appropriate SBR activated sludge methodology can provide. -a simple means of maintaining a best appropriate -. efficient approach for maintaining appropriate effluent quality standards in an energy efficient treatment system. SBRs are also. an - easy retrofit add on to pre-existing UASB reactors as a means of improving effluent quality i.e. a tertiary level effluent quality with a secondary level wastewater treatment. Keywords: post-treatment of UASB effluent, SBR, Nitrification inhibition, Bench Scale studies. iv