The sample LY-411575 was monitored by a Hach pH metre. The TS, VS, SCOD, and TCOD analyses were based on the standard methods. TN and TP in the supernatant were measured with the alkaline potassium persulphate digestion-UV spectrophotometric method and the ammonium molybdate spectrophotometric method (Zhang et al., 2009). The TOC was measured by an Elemetar (Hanau, Germany) Vario TOC analyser system.
The heavy metal concentrations were detected by inductively coupled plasma optical emission spectrometry (ICP-OES) (Perkin Elmer optima 8300DV). The supernatant samples were acidified to pH-2 and then digested in a MARS microwave digester for 30 min before ICP-OES measurement. The particle size distribution was determined via a Mastersizer 2000 particle counting system.
2.5. Acute toxicity assay
Sludge samples after anaerobic digestion were frozen and dried into powder, and 1 g of sludge (dry weight) was extracted with 10 mL of dimethyl sulfoxide (DMSO; Tedia, USA) and 10 mL of water, respectively. The mixture was shaken vigorously, sonicated for 15 min, shaken again, sonicated for a further 15 min (Bundy et al., 2001), then decanted into a glass centrifuge tube, and centrifuged at 3000 rpm for 30 min to separate suspended soil particles. The supernatant obtained was decanted into glass vials and stored at 4 °C. The extracts were used for biological tests within 24 h.