Anaerobic co-digestion of sewage and brewery sludge | |
| Author | Athapol Pecharaply |
| Call Number | AIT Diss. no.EV-07-01 |
| Subject(s) | Sewage disposal plants--Thailand--Bangkok Brewery waste--Thailand--Bangkok |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering. in Environmental Engineering and Management |
| Publisher | Asian Institute of Technology |
| Abstract | There are twelve small plants and seven central wastewater treatment plants (CWWTPs) currently operating in the Bangkok Metropolitan Region (BMR) with a total treatment capacity of more than two million m3/day, and daily sewage sludge production of 500 m3 . The raw sewage sludge sometimes is contaminated with toxic materials such as heavy metals and pathogens. It is considered not appropriate for treatment by biological treatment process, whereas these pollutants can be harmful to both humans and the environment if it is reused without appropriate treatment. The triggering potential environmental problems linked to sewage sludge treatment and its reuse, and depletion of fossil fuels have fostered the need of biological treatment of sewage sludge, the anaerobic digestion. Co-treatment by anaerobic digestion is considered as innovative and attractive technology for combined waste stabilization with significant mass and volume reduction with the generation of valuable by-products such as biogas and fertilizer. This research investigates the operating parameters and treatment efficiency for the digestion of sewage and brewery sludge. The prime objective of this study was to enhance the quality of treated sludge for use as agriculture fertilizer and to enhance biogas production, a by-product, which can be used as an energy source. Phase One: three bench-scale completely stirred tank reactor (CSTR) anaerobic digesters were operated at mesophilic condition (36±0.2°C). A mixture of sewage and brewery sludge were used as substrates at ratios of 100:0, 75:25, 50:50, 25:75 and 0:100, based on wet weight basis (w/w). For each digester, the solids retention times (SRT) were 20 days. The organic loading and volatile solids loading were between 1.3-2.2 kg chemical oxygen demand (COD)/m3 /day and 0.9-1.5 kg/m 3 /day, respectively. The digester fed with brewery sludge as co-substrate yielded higher treatment efficiency than sewage sludge alone. The removal efficiencies measured in terms of soluble chemical oxygen demand (SCOD) and total chemical oxygen demands (TCOD) ranged from 40% to 75% and 22% to 35%, respectively. Higher SCOD and TCOD removal efficiencies were obtained when higher fractions of brewery sludge were added to the substrate mixture. Removal efficiency was lowest for sewage sludge alone. Measured volatile solid (VS) reduction ranged from 15% to 20%. Adding a higher fraction of brewery sludge to the mixture increased the VS reduction percentage. The biogas production and methane yield also increased with increase in brewery sludge addition to the digester mixture. The methane content present in biogas of each digester exceeded 70% indicating the system was functioning as an anaerobic process. Likewise the ratio of brewery sewage influenced not only the treatment efficiency but also improved the quality of treated sludge by. lowering the number of pathogen (less than 2 MPN/g of dried sludge) and maintaining a high nutrient concentration of nitrogen (N) 3.2-4.2%, phosphorus (P) 1.9-3.2% and potassium (K) 0.95- 0.96%. The heavy metals, chromium (Cr) and copper (Cu) remaining in ,digested sludge were present at relatively high levels (Cr 1,849-4,230 and Cu 930-2,526 mg/kg dried sludge). However, the metals were presented as organic matter bound and sulfide bound fractions, which are not soluble and available. The digested sludge could be safely applied to soil as a plant nutrient source, without fecal coliforms or heavy metals risk. A sludge mixture ratio of 25:75 (sewage:brewery) which generated the higher nutrient concentrations (N=4.22%, P=3.20% and K=0.95%), biogas production and treatment efficiency met the Bangkok Metropolitan Administration (BMA) safety guidelines required for agricultural application. Biogas production and methane at the 25:75 ratio (sewage: brewery) yielded highest amount of VSremoved (0.65 m3/kg) and CODremoved (220 L/kg) respectively. Phase Two: Ni was spiked into sewage sludge to its highest level ever found in the BMA records at 1200 mg/kg while co-treating at various ratios under the same controlled environmental conditions as previous experiment. The main purpose was to study the effects of the treatment system from treating high heavy metal contaminated sludge. At the ratio of 25:75, it showed the highest treatment efficiency of TCOD, SCOD, and VS reduction at 26%, 62%, and 18%, respectively. Daily Biogas production equaled 1.76 L/day whilst, cumulative biogas during full period of operation was 162L. The biogas had high methane level at 72.1 %, whereas biogas production rate and methane yield were found at 0.61 m3/kg VSremoved and 220 L/kg CODremoved, respectively. Digested sludge contained high nutrients and low pathogens (fecal coliforms). As for the heavy metals (Cr and Cu) in digested sludge was considered higher than the BMA guideline and organic fertilizer standards of DOA. However, this experiment showed that treating high Ni (3.5 times higher than background level) sludge mixtures resulted in decreased in 'both daily biogas production and total biogas production ranged between 9-18%. Moreover, the treatment efficiencies of TCOD, SCOD and VS reduction all decreased at 7.4-13.4%, 11- 17%, and 15.4-29%, respectively. It can be seen clearly that Ni is a one of limiting factor in suppressing the growth and development of microorganisms in the sludge digester under its mesophilic condition. · From this study, the sampled brewery sludge that was obtained had a unexpected high content of heavy 'metals especially cadmium, which in normal situation should not have much concentration, thereby, producing digested sludge higher than permitted by BMA and DOA standards for use as fertilizer. However, it was found that co-digestion between sewage sludge and brewery sludge enhanced digestion efficiency, biogas production including improvement in treated sludge quality. |
| Year | 2007 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Energy and Climate Change (Former title: Department of Energy, Environment, and Climate Change (DEECC)) |
| Academic Program/FoS | Environmental Engineering and Management (EV) |
| Chairperson(s) | Preeda Parkpian ;Annachhatre, Ajit P. ; |
| Examination Committee(s) | Eckhardt, Heinz ;Sutat Weesakul ;DeLaune, Ronald D. ; |
| Scholarship Donor(s) | AIT Fellowship; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2007 |