Anaerobic Treatment of Domestic Wastewater in Small Scale UASB Reactors

Abstract

A survey was made on the feasibility of on-site anaerobic treatment of domestic wastewater from small scale emissions in The Netherlands. Three 1.2 m3 UASB-reactors (Upflow Anaerobic Sludge Blanket) were tested in different rural locations. The survey made clear that the efficiency of the process is highly dependent on the reactor temperature. Below 12 °C purification was predominantly based on settling, while above 12 °C microbial degradation of organic matter increased. Efficiencies for the removal of total COD and BOD of two of the reactors ranged between 33 and 60%, equalising well performing septic tanks. In one of the reactors the average production of biogas, with a methane content of 78%, was 67 liters per day. Complete transformation of volatile fatty acids (VFA) into biogas was achieved during 3 to 4 months a year at temperatures above 15 °C. In this period, during the second year of operation, the average efficiencies for COD and BOD removal were 60 and 72% respectively, while the maximum gas production reached 300 liters per day. In the latter period the mineralisation rate exceeded the input rate of organic matter, resulting in a decrease of settled material in the reactor. The possibility of high efficiencies during summertime in The Netherlands and the positive results of similar experiments in Indonesia and Latin America lead to the conclusion that anaerobic digestion is a promising process for the (pre-)treatment of domestic waste water in (sub)tropical countries.