【正文】 ture during the period of investigation was in the range of 30–35 _C. Treatment pondsA schematic of waste stabilization pond system (WSPS) of the starch factory is presented in Fig. 1 (Choi, 2001). The wastewater treatment system consists of 21 APs and facultative ponds (FPs) connected in series with total area of about ha. Out of these, 6 are AP with area of ha and 15 are FP with ha. The study concentrated on anaerobic ponds system. During the study period only four anaerobic ponds were in operation. The typical size of an AP is approximately 250 m in length, 100 m in width and 4–5 m in depth.The pond parameters are presented in Table 1. The anaerobic ponds treat wastewater from a starch as well as from a glucose factory. The wastewater from the starch factory was first introduced to Pond 2 and subsequently flows to Pond 4 while the effluent fromthe glucose factory was introduced to Pond 3 and then flows to Pond 5 where wastewater from the starch and glucose factory were bined. The bined wastewater then flows into a series of FPs and treated effluent was finally discharged on to the groundwater recharge spreading basins. Sludge activity testsThe schematic of the SMA test set up is presented in Fig. 2. To determine SMA, a known amount of sludge obtained from the sediment layer of Pond 4 was transferred into serum bottles (115 ml) after washing three times with water to remove existing COD. While100 ml of bulk liquid from the same pond was kept in a serum bottle to determine SMA of sludge in suspension in the bulk liquid layer. An appropriate amount of starch factory wastewater as substrate was added to the serum bottles so as to obtain an initial COD level in the range of 2000–2500 mg/l. Nutrients were added to maintain a carbon:nitrogen:phosphorus ratio of 300:5:1. pH was adjusted to between 7 and . Two grams per liter of NaHCO3 were also added along with substrate to buffer the serum bottle contents to near neutral pH conditions during the test. Subsequently the bottles weresealed with a rubber septum and an aluminum cap after purging oxygen with nitrogen (N2) gas and attached to the liquid displacement system. The liquid displacement bottle contained 3% NaOH solution. Methane gas production was measured at different time intervals up to 48 h. After every gas measurement, by swirling manually, the contents of the serum bottle were mixed. The tests were conducted in a 30 _C temperaturecontrolled room. Likewise, cyanide degradation activity of anaerobic pond sediment was carried out also in the serum bottles. A known amount of sludge obtained from Pond 4 sediment layer was kept in serum bottles and filled with 70 ml of wastewater having substrate andnutrient similar to those mentioned for the SMA test. A stock cyanide solution was added into each serum bottle to achieve cyanide concentrations of 10 and 20 mg/l respectively. The bottle was then purged with N2 gas and immediately sealed with a rubber septum and an aluminum cap. The bottle was kept in a 30 _C temperature controlled room. Samples were taken with Hamilton syringe at every 8 h interval for 48 h and analyzed for cyanide content.. Rajbhandari, . Annachhatre / Bioresource Technology 95 (2004) 135–143 Suspended solid settling experimentBatch tests to investigate TSS settling characteristics of starch factory wastewater under quiescent condition was carried out in a settling column of cm diameter and m height for different TSS concentrations. Wastewater of a desired TSS concentration for settlingexperiment was prepared by diluting the concentrated wastewater with tap water. The wastewater was poured into a settling column after stirring thoroughly. Samples from top of column were collected at different time intervals ranging from 2 to 60 min and analyzed for TSS concentration. Analytical proceduresParameters including COD, BOD5, TSS, volatile suspended solid (VSS) and dissolved solid (DS) were analyzed according to Standard Methods (APHA et al., 1998). The mass of the sludge used in the sludge activity test was measured in terms of VSS. All the samples were filtered through lm glass fiber filters for the determination of soluble COD and BOD5. Cyanide was measured spectrophotometrically (Spectroquant, E. Merck KGaA, Darmstadt, Germany) as per the procedurereported elsewhere (Annachhatre and Amornkaew, 2000). Statistical analysisThe anaerobic ponds process performance data were presented in terms of arithmetic averages of nine values 177。standard deviation. The SMA tests were carried out with two replicates. Common linear regression curve was fitted to the data obtained from the two replicates test and a relation between volumes of methane production with respect to time was established. The SMA was calculated based on the slope of methane volume versus time curve and mass of sludge taken for the SMA test. Likewise a linear relation was established between the cumulative cyanide degradation with respect to time. Data from settling experiments was used to establish a nonlinear relationship between the halfremoval time and influent total suspended solid concentrations. All statistical analyses (arithmetic average, standard deviation, linear and nonlinear regression and correlation coefficient) were performed using Microsoft Excel 2000.3 Results and discussion Analysis of existing wastewater process Characteristics of raw wastewater: The pond system treats approximately 4500 m3/d of wastewater from starch and approximately 500 m3/d of wastewater from glucose factory. A scheme of anaerobic ponds and sampling points is shown in Fig. 3. The characterization of raw, influent and effluent wastewater of the pond systems is shown in Table 2. The wastewater characteristics at sampling point, ‘a(chǎn)’ and ‘d’, in Table 2 corresponds, respectively to raw wastewater from starch and glucose factory. Raw wastewater from starch factory was highly acidic in natu