Multi stage anaerobic reactor for wastewater treatment

Abstract

Staged reactors (two or more stages) in anaerobic digestion (AD) provide a solutions to meet wastewater requirements. Compared with other AD treatment options, staging can more effectively adapt to toxic slugs and improve its treatment efficiency by passing them in less time through the reactor system. In this study, the Multi Stage Anaerobic Reactor (MSAR) was developed and tested with various fundamental experiments, such as start-up performance, ability to withstand low hydraulic retention time (HRT) and high organic loading rate (OLR), generation of biomethane recovery, and ability to tolerate inhibitory compound (tetracycline and papermill wastewater). The start-up process was commenced with a synthetic wastewater (glucose) with an OLR of 0.18 kg COD/m3/d at HRT of 4 d, and accomplished acclimatization and stable operation for the subsequent phase of the study. The effect of HRT on the process performance of the MSAR was investigated by gradually reducing the HRT from 4 d to 1 d. Results showed that not much difference in terms of COD removal performance. It was observed that when HRT was reduced from 4 d to 1 day, COD removal achieved was from 99.4% to 91.9%, signifying the stability of MSAR at low HRT. Meanwhile, volatile acid (VA) of the MSAR effluent fluctuated from 13 to 276 mg/L as HOAc, confirming no great variations in the profile. A variation ratio (VSS/TSS) from 0.2 to 0.92 in all stages was observed confirming no significant sludges wash out during this phase and the highest generation of methane observed at 0.0153 LCH4/g COD for HRT 4 d in stages. A different scenario was observed during the study of OLR effect on MSAR. The COD removal efficacy declined dramatically from 91% to 55%, 50%, 47% and 16%, when the OLR was increased from 0.79 kg COD/m3/d to 1.05, 1.31, 1.57 and 1.84 kg COD/m3/d, respectively. Meanwhile, the average of VA concentration escalated from 306 mg/L to 883, 1066, 1672 and 1581 mg/L as HOAc in the effluent stage (S4). The ratio of VSS/TSS in this study showed a value between 0.31-0.91 in stages. Concurrently, a decreased in methane generation was also observed at 0.0235, 0.0105, 0.0087, 0.0047 and 0.0045 LCH4/g COD in stages. During the study on the effect of inhibitory substances to MSAR, tetracycline was greatly deteriorated the performance of the reactor. At 10 mg/L tetracycline concentration, the COD performance was dropped to 12.33% removal. At the same time, when the papermill wastewater tested, an average COD reduction of 75.8% was observed in this phase. The lethal effect of tetracycline significantly determines the abundance of VA result (2700 mg/L as HOAc), which leads to the inhibition of methane production in stages. An acceptable value of VA (= 500 mg/L as HOAc) was observed during the effect of papermill wastewater confirming a potential tolerance to methanogenic microorganisms in this phase. Meanwhile, two theoretical methods for methanogen potential was carried out in this work. These methods use the COD characterization and elemental composition analysis corresponding to the affected parameters (HRT, OLR, tetracycline, and papermill wastewater) to predict methane production rates in stages. The COD characterization analysis observed the highest methane potential at 0.021 LCH4/g COD for 3 d HRT, 0.06 LCH4/g COD at OLR 0.785 kg COD/m3/d, 0.054 LCH4/g COD at 0.13 mg/L tetracycline concentration and 0.05 LCH4/g COD at OLR of 0.785 kg COD/m3/d papermill wastewater in stages, respectively. By elemental composition analysis, the highest methane observed was 0.25 LCH4/g COD at 3 d HRT, 0.197 LCH4/g COD at 1.571 kg COD/m3/d and 0.253 LCH4/g COD at 1.07 mg/L tetracycline concentration in the stages. Therefore, this study offers a potential solution for the use of MSAR to treat wastewater and contributes to the development of anaerobic reactor technology across Malaysian industries.