High-rate anaerobic treatment of dilute wastewater at psychrophilic temperature

Abstract

<p>The application of anaerobic technology has been mostly directed towards the treatment of medium and high strength wastewater at mesophilic and thermophilic temperatures. The possibilities of anaerobic treatment could be increased if the process could be applied at low temperatures for treatment of various types of dilute wastewater. Problems limiting the treatment of dilute wastewater at psychrophilic temperature are related to the wastewater and the reactor design;This research evaluated the application of the anaerobic sequencing batch reactor (ASBR) at various low temperatures and hydraulic retention times (HRTs). Anaerobic treatment of dilute wastewater was studied using three, laboratory-scale ASBRs, each with an active volume of six (6) liters. The reactors were fed a synthetic substrate made from non-fat dry milk (COD = 600 mg/L and BOD5 = 285 mg/L) supplemented with nutrients and trace metals. Pseudo steady-state performance data were collected at reactor temperatures of 25, 20, 17.5, 15, 12.5, 10, 7.5 and 5°C over a period of two years. HRTs were maintained at 24, 16, 12, 8 and 6 hours. Results showed that the ASBR process was capable of achieving in excess of 90% soluble COD and BOD5 removal at temperatures of 25°C and 20°C at all HRTs. At the low temperature of 5°C and the six hour HRT, soluble COD and BOD5 removals were 62% and 75%, respectively. In all cases, solid retention times (SRTs) were high enough to maintain good performance;In addition, the methanogenic activity, microstructure and size distribution of the ASBR granules were also evaluated. Granules at 15 and 25°C exhibited uniform structure, predominantly Methanothrix-like microorganisms, while granules at 5°C indicated the existence of a layered structure. The equivalent diameter of granules was 2.0-3.3 mm for all temperatures and HRTs;This research also evaluated the effects of low temperatures on the microbial kinetic parameters K, Ks, Kd, Yg and [mu]m. The values of K, Kd and [mu]m were found to decrease, and the half saturation constant, Ks was found to increase with the decrease of temperature in accordance with the Arrhenius equation. Temperature coefficient, [theta] values for K, [mu]m, Kd and Ks were 1.083, 1.062, 1.076 and 1.05 respectively indicating a linear interrelationship among them. Through the determination of kinetic parameters, a design engineer may be able to predict the performance of the treatment systems of ASBR over a wide range of psychrophilic temperatures.</p>