An aerobic reactor (2.5 L) was operated as a continuous system for a period of 700 days. The reactor was continuously fed with Kraft E1 effluent (at pH 7) by means of a peristaltic pump. The hydraulic retention time (HRT) was maintained in 12 h. The sludge was periodically recycled, and excess sludge which grew during the aeration stage was withdrawn from the bottom to obtain solids retention times (SRT) in 10 days. Inside the reactor, the oxygen concentration was maintained above 5 mg L-1 by an aquarium-type air pump with sintered-sand diffusers at the bottom of the reactor. The adequate nutrient balance is necessary to obtain sufficient biodegradation. An estimate of requirements for N and P is based on the fact that wasted mixed liquor volatile suspended solids (MLVSS) contains approximately 12% of its dry weight, as N and 2% as P [39]. N as urea and P as KH2PO4 were
supplied to maintain a C:N:P ratio of 100:5:1. Dissolved oxygen, pH, temperature, total suspended solids and volatile suspended solids were regularly monitored and adjusted when necessary [31,32].