環(huán)境科學(xué)與工程畢業(yè)設(shè)計(jì)文章(英文版)

Anaerobic ponds treatment of starch wastewater: case study in Thailand B.K. Rajbhandari, A.P. Annachhatre * Environmental Engineering and Management, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand Received in revised form 20 January 2004; accepted 26 January 2004 Available online 12 March 2004 Abstract Anaerobic ponds are particularly eff ective in treating high-strength wastewater containing biodegradable solids as they achieve the dual purpose of particulate settlement and organic removal. Performance of an anaerobic pond system for treatment of starch wastewater containing high organic carbon, biodegradable starch particulate matter and cyanide was assessed under tropical climate conditions. Approximately 5000 m3/d of wastewater from starch industry was treated in a series of anaerobic ponds with a total area of 7.39 ha followed by facultative ponds with an area of 29.11 ha. Overall COD and TSS removal of over 90% and CN removal of 51% was observed. Active biomass obtained from the anaerobic ponds sediments and bulk liquid layer exhibited specifi c methano- genic activity of 20.7 and 11.3 ml CH4/g VSSd, respectively. The cyanide degradability of sludge at initial cyanide concentration of 10 and 20 mg/l were determined to be 0.43 and 0.84 mg CN?/g VSSd, respectively. A separate settling column experiment with starch wastewater revealed that a settling time of approximately 120 min is suffi cient to remove 9095% of the infl uent TSS. ? 2004 Elsevier Ltd. All rights reserved. Keywords: Anaerobic pond; Cyanide degradability; Organic carbon; Settling characteristics; Specifi c methanogenic activity; Starch factory wastewater 1. Introduction Anaerobic ponds (APs) are popularly employed for treatment of organic wastewater emanating from variety of industries such as food, pulp and paper, sugar and distillery. Anaerobic ponds are particularly eff ective in treating high-strength wastewaters containing biodegrad- able total suspended solids (TSS). In such cases the liquid layer in anaerobic ponds act as a settling basin for the suspended solids while the anaerobic biodegradation primarily takes place in pond sediments (Toprak, 1994). Anaerobic reactions taking place in the sediment include solubilization of biodegradable particulate matter fol- lowed by acidogenesis, acetogenesis and methanogenesis (Parker, 1979; Pescod, 1996). The reactions occurring in the bulk liquid are often negligible as compared to those in the pond sediments. Thus, anaerobic ponds achieve a dual purpose of sedimentation of particulate matter as well as anaerobic conversion of organics. However, anaerobic pond operation also has many intrinsic prob- lems such as high land requirements and emission of obnoxiousandgreenhousegasessuchashydrogensulfi de (H2S),carbondioxide(CO2)andmethane(CH4)(Parker, 1979; Pescod, 1996; Toprak, 1997; Paing et al., 2003). In spite of these problems, anaerobic ponds are popular particularly wherever land is abundant (Arthur, 1983). Wastewater coming from starch factories is one such type of wastewater, which is treated extensively in anaerobic ponds. Starch is often produced in many parts of the world from tapioca. Tapioca roots contain 20 25% starch. The starch extraction process essentially involves pre-processing of roots, followed by starch extraction, separation and drying. The process generates 2060 m3/ton of wastewater with a low pH in the range 3.85.2 (Economic and Social Commission for Asia and The Pacifi c, 1982). The wastewater is highly organic in nature with chemical oxygen demand (COD) up to 25,000 mg/l (Bengtsson and Treit, 1994). The wastewater consists of high TSS comprising starch granules in the range 300015,000 mg/l, which are highly biodegradable by nature. Tapioca starch wastewater also has high cyanide content up to 1015 mg/l, which is highly toxic *Corresponding author. Tel./fax: +66-2-524-5644. E-mail address: ajitait.ac.th (A.P. Annachhatre). 0960-8524/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.01.017 Bioresource Technology 95 (2004) 135143 to aquatic life at concentrations of cyanide as low as 0.3 mg/l have been reported as cause for a massive fi sh kill (Bengtsson and Treit, 1994). Problems related to water pollution are reported to be serious. The acidic nature of wastewater can harm aquaticorganismsandreducethe self-purifi cation capacity of the receiving stream. Suspended solids present in the wastewater can settle on the streambed and spoil fi sh breeding areas in the stream. Since these solids are primarily organic in nature, they decompose easily and thus deoxygenate the water. Similarly, high biochemical oxygen demand (BOD) of the wastewater also can cause rapid depletion of oxygen content in the receiving water body and promote the growth of nui- sance organisms. Water pollution caused by tapioca starch production has been reported as a serious prob- lem in many Asian countries, particularly in Thailand (Kiravanich, 1977) and in India (Padmaja et al., 1990). Tapioca also contains bound cyanide as a natural de- fense mechanism. During the starch manufacturing process, bound cyanide in the form of linamarin and lotaustralin from tapioca roots is hydrolyzed by the enzyme linamarase with decomposition to hydrogen cyanide (HCN), which fi nds its way into the wastewater. Cyanide containing starch wastewater can be eff ectively detoxifi ed in anaerobic processes (Annachhatre and Amornkaew, 2000). Upfl ow anaerobic sludge blanket (UASB) processes are eff ective in treating starch waste- water (Annachhatre and Amatya, 2000), particularly, in removingcyanide(AnnachhatreandAmornkaew, 2001). Adaptation by methanogens to cyanide concen- trations of 530 mg/l has been reported in literature (Fedorak et al., 1986; Harper et al., 1983). Thus, in treating tapioca starch wastewater anaerobic ponds achieve a threefold objective namely: sedimentation of particulate matter, anaerobic conversion of organics and detoxifi cation of cyanide. Accordingly, the work presented here assesses the performance of APs treating wastewater from tapioca starch industry, particularly related to COD, TSS and cyanide removal. Since APs serve as a settling basin for starch granules, the settling characteristics were also assessed by column experiments. Furthermore, the po- tential methane production rates of anaerobic biomass (sludge) obtained from the AP sediment as well as from bulk liquid layer were assessed from the specifi c meth- anogenic activity (SMA) test. The cyanide degradability of the anaerobic sludge obtained from the pond sedi- ment layer was also assessed. 2. Methods Investigations on the existing wastewater anaerobic pond system were carried out in a tapioca starch and glucose factory situated in the Central province of Thailand with a capacity of 250 tons starch/day. The factory uses groundwater as a source for process water, and generates combined wastewater of approximately 5000 m3/d. The operating ambient temperature during the period of investigation was in the range of 3035 ?C. 2.1. Treatment ponds A 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 36.5 ha. Out of these, 6 are AP with area of 7.39 ha and 15 are FP with 29.11 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 45 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 fi rst introduced to Pond #2 and subsequently fl ows to Pond #4 while the effl uent from the glucose factory was introduced to Pond #3 and then fl ows to Pond #5 where wastewater from the starch and glucose factory were combined. The combined waste- water then fl ows into a series of FPs and treated effl uent was fi nally discharged on to the groundwater recharge spreading basins. 2.2. Sludge activity tests The 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. While 100 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 20002500 mg/l. Nutrients were added to maintain a carbon:nitrogen:phosphorus ratio of 300:5:1. pH was adjusted to between 7 and 7.8. Two grams per liter of NaHCO3were also added along with substrate to buff er the serum bottle contents to near neutral pH conditions during the test. Subsequently the bottles were sealed 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 diff erent 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 temperature- 136B.K. Rajbhandari, A.P. Annachhatre / Bioresource Technology 95 (2004) 135143 controlled 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 fi lled with 70 ml of wastewater having substrate and nutrient 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 N2gas and immediately sealed with a rubber septum and an aluminum cap. The bottle was kept in a 30 ?C temper- ature-controlledroom.Samplesweretakenwith Hamilton syringe at every 8 h interval for 48 h and analyzed for cyanide content. 2.3. Suspended solid settling experiment Batch tests to investigate TSS settling characteristics of starch factory wastewater under quiescent condition Fig. 1. Layout of WSPS of starch factory. Table 1 Ponds parameters APsSurface area (ha)Volume (m3104)Depth (m) Infl owb(m3/d)Detention timeb(d) 1a1.074.414.5 21.345.574.5450173112.71.9 32.068.624.549882177.832.8 41.767.344.5450173116.72.5 50.491.784.549997853.60.5 6a0.672.464.5 aPond not in operation at the time of this study. bResults are mean of nine valuesstandard deviation. Fig. 2. Sludge activity test setup. B.K. Rajbhandari, A.P. Annachhatre / Bioresource Technology 95 (2004) 135143137 was carried out in a settling column of 10.0 cm diameter and 2.0 m height for diff erent TSS concentrations. Wastewater of a desired TSS concentration for settling experiment 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 diff erent time intervals ranging from 2 to 60 min and analyzed for TSS concentration. 2.4. Analytical procedures Parameters 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 fi ltered through 0.45 lm glass fi ber fi lters for the deter- mination of soluble COD and BOD5. Cyanide was measuredspectrophotometrically(Spectroquant,E. Merck KGaA, Darmstadt, Germany) as per the proce- dure reported elsewhere (Annachhatre and Amornkaew, 2000). 2.5. Statistical analysis The anaerobic ponds process performance data were presented in terms of arithmetic averages of nine val- uesstandard deviation. The SMA tests were carried out with two replicates. Common linear regression curve was fi tted to the data obtained from the two replicates test and a relation between volumes of methane pro- duction 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 non-linear relationship between the half-removal time and infl uent total suspended solid concentrations. All statistical analyses (arithmetic average, standard devia- tion, linear and non-linear regression and correlation coeffi cient) were performed using Microsoft Excel 2000. 3. Results and discussion 3.1. 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, infl uent and effl uent wastewater of the pond systems is shown in Table 2. The wastewater charac- teristics at sampling point, a and d, in Table 2 cor- responds, respectively to raw wastewater from starch and glucose factory. Raw wastewater from starch fac- tory was highly acidic in nature while from glucose factory was low acidic to neutral. As can be seen in Table 2, the major pollution load was due to wastewater from starch factory having BOD5of 12,776499 mg/l as compared to BOD5of 1046153 mg/l from glucose factory. The starch factory wastewater also had TSS of 91303067 mg/l mainly as starch granules, which were highly biodegradable by nature. A cyanide concentra- tion of 17.51.5 mg/l was found in starch factory wastewater while no cyanide was detected in wastewater from glucose factory. Performance of anaerobic ponds: The details of the pond area and residence time are presented in Table 1. The overall residence time works out to be 335 days for starch and 18133 days for glucose factory wastewater. The average pollution load for the total wastewater fl ows of 4999785 m3/d calculated to be 63,25810,198 kg COD/d with 62,73210,152 kg COD/d from starch and 658138 kg COD/d from glucose factory. The average overall volumetric loading in the anaerobic ponds was 49782 kg BOD5/m3d (51482 kg COD/m3d). abc de f Anaerobic ponds Sampling point: a.Starch wastewater/influent to pond 2 b. effluent from pond 2/ influent to pond 4 c.effluent from pond 4/ influent to pond 5 d.Glucose wastewater/ influent to pond 3 e.effluent from pond 3/ influent to pond 5 f.effluent from pond 5/ influent to pond 6 1 Starch wastewater Glucose wastewater 2 3 4 5 6 Fig. 3. Scheme of anaerobic ponds and sampling points. 138B.K. Rajbhandari, A.P. Annachhatre / Bioresource Technology 95 (2004) 135143 Out of six anaerobic ponds, Ponds #1 and #6 were not in operation during the study period. Pond #1 was fi lled up due to accumulation of starch granules from starch wastewater so wastewater from the starch factory was introduced into Pond #2. The average COD, BOD5 and TSS removal in Pond #2 is very small, about 10.56.8%, 8.66.2% and 18.010.9%, respectively (Table 3). It was observed that Pond #2 was also par- tially fi lled up by starch granules and a channel was formed where wastewater fl owed to Pond #4. This indicates that Ponds #1 and #2 operate mainly as set- tling basins for the suspended solids, and hence, they need to be desludged regularly. Accumulation of starch granules in the pond also reduces the residence time in the pond signifi cantly. The pH in Pond #2 was acidic, in the range 4.14.3. Under this condition, methanogenesis cannot occur, as this condition is highly unfavorable for the growth of methanogenic bacteria (Duarte and Anderson, 1982). This is further brought out by the fact that BOD5re- moval in Pond #2 was less than 10%. However, in Ponds #4 and #5, the pH was between 6 and 8 as these ponds were active anaerobically. In fact, intense biological activity was observed in these two ponds as evidenced by formation of excessive gas bubbles and the existence of fl oating sludge on the pond surface. According to Zehnder et al. (1982), the optimum pH range for all methanogenic bacteria is between 6.0 and 8.0, but the optimum value for the group as a whole is close to 7.0. Van Haandel and Lettinga (1994) reported the same observation. Based on data in Table 3, it is apparent that the performance of Ponds #4 and #5 is satisfactory. Pond #4 was the most effi cient one and provided average COD, BOD5and TSS removal of 88.60.6%, 90.5 0.6% and 87.62.8%, respectively. The average volu- metric loading of 1031165 g BOD5/m3d in Pond #2 was very high and 62 g BOD5/m3d in Pond #3 was very low, while 716128 and 30047 g BOD5/m3d in Ponds #4 and #5, respectively, were within the range found in much of the literature (Ellis, 1980; Arthur, 1983; Gomes de Sousa, 1987; Mara and Pearson, 1998). The starch wastewater also contained 17.51.5 mg/l of cyanide. Since the ponds have been in operation for over 20 years, it was anticipated that the sludge would be well acclimatized to cyanide present in the wastewater. Average cyanide removal of 2.82.5%, 38.42.6% and 9.25.0% was observed in anaerobic Ponds #2, #4 and #5, respectively. The overall removal rate for COD, BOD5and TSS were 96.20.6%, 98.20.4% and 94.71.3% respec- tively (Table 3), whereas the removal effi ciencies for DS and CN were 71.41.0% and 51.21.1%, respectively. However, the quality of treated effl uent from the series of anaerobic ponds (Table 3, corresponding to sampling point f) still did not meet the