W. Charles

Effect of pre-aeration and inoculum on the start-up of batch thermophilic anaerobic digestion of municipal solid waste.

In this study, a short pre-aeration step was investigated as pre-treatment for thermophilic anaerobic digestion of the organic fraction of municipal solid waste (OFMSW). It was found that pre-aeration of 48 h generated enough biological heat to increase the temperature of bulk OFMSW to 60 degrees C. This was sufficient self-heating of the bulk OFMSW for the start-up of thermophilic anaerobic digestion without the need for an external heat source. Pre-aeration also reduced excess easily degradable organic compounds in OFMSW, which were the common cause of acidification during the start-up of the batch system. Careful consideration however must be taken to avoid over aeration as this consumes substrate, which would otherwise be available to methanogens to produce biogas. To accelerate methane production and volatile solids destruction, the anaerobic digestion in this study was operated as a wet process with the anaerobic liquid recycled through the OFMSW. Appropriate anaerobic liquid inoculum was found to be particularly beneficial. It provided high buffer capacity as well as suitable microbial inoculum. As a result, acidification during start-up was kept to a minimum. With volatile fatty acids (VFAs-acetate in particular) and H2 accumulation typical of hydrolysis and fermentation of the easily degradable substrates during start-up, inoculum with high numbers of hydrogenotrophic methanogens was critical to not only maximise CH4 production but also reduce H2 partial pressure in the system to allow VFAs degradation. In a lab-scale bioreactor, the combined pre-aeration and wet thermophilic anaerobic digestion was able to stabilise the OFMSW within a period of only 12 days. The stabilised inert residual material can be used as a soil amendment product.

Comparison of static, in-vessel composting of MSW with thermophilic anaerobic digestion and combinations of the two processes

The biological stabilisation of the organic fraction of municipal solid waste (OFMSW) into a form stable enough for land application can be achieved via aerobic or anaerobic treatments. To investigate the rates of degradation (e.g. via electron equivalents removed, or via carbon emitted) of aerobic and anaerobic treatment, OFMSW samples were exposed to computer controlled laboratory-scale aerobic (static in-vessel composting), and anaerobic (thermophilic anaerobic digestion with liquor recycle) treatment individually and in combination. A comparison of the degradation rates, based on electron flow revealed that provided a suitable inoculum was used, anaerobic digestion was the faster of the two waste conversion process. In addition to faster maximum substrate oxidation rates, anaerobic digestion (followed by post-treatment aerobic maturation), when compared to static composting alone, converted a larger fraction of the organics to gaseous end-products (CO2 and CH4), leading to improved end-product stability and maturity, as measured by compost self-heating and root elongation tests, respectively. While not comparable to windrow and other mixed, highly aerated compost systems, our results show that in the thermophilic, in-vessel treatment investigated here, the inclusion of a anaerobic phase, rather than using composting alone, improved hydrolysis rates as well as oxidation rates and product stability. The combination of the two methods, as used in the DiCOM process, was also tested allowing heat generation to thermophilic operating temperature, biogas recovery and a low odour stable end-product within 19 days of operation.

Recovery of sulphur from contaminated air in wastewater treatment plants by biofiltration: a critical review

AbstractBiofilters are popular as an alternative method for treatment of volatile air pollutants like hydrogen sulphide originating from wastewater treatment plants. Despite several advantages over conventional chemical systems, one of the concerns of biological treatment of hydrogen sulphide is the production of large volumes of neutral or acidic leachate which needs to be treated or disposed safely. Instead of treating as an unwanted product, a waste stream of weakly acidic leachate can be thought of as a sulphur resource. This paper provides an overview of recent literature on the removal of H2S from contaminated air in an aerobic environment and discusses the possibility of recovering sulphur from contaminated air with special emphasis on polluted air originating from wastewater treatment plants. We also add our perspectives on future research and development needs in this area.

Enhancement of waste activated sludge anaerobic digestion by a novel chemical free acid/alkaline pretreatment using electrolysis

Anaerobic digestion of waste activated sludge (WAS) is relatively poor due to hydrolysis limitations. Acid and alkaline pretreatments are effective in enhancing hydrolysis leading to higher methane yields. However, chemical costs often prohibit full-scale application. In this study, 12 V two-chamber electrolysis using an anion exchange membrane alters sludge pH without chemical dosing. pH dropped from 6.9 to 2.5 in the anode chamber and increased to 10.1 in the cathode chamber within 15 h. The volatile suspended solids solubilisation of WAS was 31.1% in the anode chamber and 34.0% in the cathode chamber. As a result, dissolved chemical oxygen demand increased from 164 to 1,787 mg/L and 1,256 mg/L in the anode and cathode chambers, respectively. Remixing of sludge from the two chambers brought the pH back to 6.5, hence no chemical neutralisation was required prior to anaerobic digestion. Methane yield during anaerobic digestion at 20 d retention time was 31% higher than that of untreated sludge. An energy balance assessment indicated that the non-optimised process could approximately recover the energy (electricity) expended in the electrolysis process. With suitable optimisation of treatment time and voltages, significant energy savings would be expected in addition to the benefit of decreased sludge volume.

The use of redox potential to monitor biochemical HCBD dechlorination

The microbial reductive dechlorination of chlorinated solvents is a redox reaction in which the chlorinated carbon receives electrons from a suitable electron donor. Hexachloro-1,3-butadiene (HCBD) is a particularly recalcitrant chlorinated solvent. Its slow, cyanocobalamin-dependent biochemical dechlorination by mixed microbial consortia had been previously demonstrated. This study shows that the reductive dechlorination reaction of HCBD can be monitored in situ by recording the redox potential (E(Ag/AgCl)). The addition of HCBD to mixed anaerobic consortia triggered a rise of E(Ag/AgCl) and the formation of dechlorinated endproducts. This indicated that the change in E(Ag/AgCl) was linked to the dechlorination of HCBD. Total concentration of dechlorinated endproducts (C4 gases) equated to approximately 50% of HCBD added. The rise and subsequent fall in E(Ag/AgCl) after the addition of HCBD caused a peak. Peak areas obtained, from the change in E(Ag/AgCl), provided an indication of the amount of HCBD dechlorinated. Moreover, the saturation concentration of HCBD can be estimated from peak heights. This online monitoring of HCBD reductive dechlorination could potentially be used for improved process control of bioremediation reactors and on-site as online biosensors. Online monitoring of HCBD dechlorination via redox potential offers both reliability and portability at a low cost. In addition, this novel and innovative technique could potentially be used to monitor the dechlorination of contaminants other than HCBD.

Novel process of bio-chemical ammonia removal from air streams using a water reflux system and zeolite as filter media

A novel biofilter that removes ammonia from air streams and converts it to nitrogen gas has been developed and operated continuously for 300 days. The ammonia from the incoming up-flow air stream is first absorbed into water and the carrier material, zeolite. A continuous gravity reflux of condensed water from the exit of the biofilter provides moisture for nitrifying bacteria to develop and convert dissolved ammonia (ammonium) to nitrite/nitrate. The down-flow of the condensed water reflux washes down nitrite/nitrate preventing ammonium and nitrite/nitrate accumulation at the top region of the biofilter. The evaporation caused by the inflow air leads to the accumulation of nitrite to extremely high concentrations in the bottom of the biofilter. The high nitrite concentrations favour the spontaneous chemical oxidation of ammonium by nitrite to nitrogen (N2). Tests showed that this chemical reaction was catalysed by the zeolite filter medium and allowed it to take place at room temperature. This study shows that ammonia can be removed from air streams and converted to N2 in a fully aerated single step biofilter. The process also overcomes the problem of microorganism-inhibition and resulted in zero leachate production.

Bioelectrochemical enhancement of anaerobic digestion: Comparing single- and two-chamber reactor configurations at thermophilic conditions

Bioelectrochemical system (BES) can act as an auxiliary technology for improving organic waste treatment and biogas production in anaerobic digestion (AD). For the first time this study directly compared the performance of a single- and a cation-exchange membrane-equipped two-chamber BES-AD systems at thermophilic conditions. The results indicated that an active glucose-fed thermophilic anaerobic sludge could readily (<3days) increase biogas production in both reactor configurations by inserting a carbon electrode poised at -0.8V (vs. Ag/AgCl). However, after a 3-week operation, the biogas production rates from the single- and two-chamber BES reactor decreased due to volatile fatty acids accumulation. Only the two-chamber configuration could enable methane enrichment (98% CH4v/v) in biogas. Overall, this study suggests that integrating bioelectrodes in-situ could not sustainably improve biogas production in a thermophilic AD reactor, and future studies should be directed towards the use of bioelectrodes for improving biogas quality.

Anaerobic acidification of sugar-containing wastewater for biotechnological production of organic acids and ethanol

ABSTRACT Anaerobic acidification of sugars can produce some useful end-products such as alcohol, volatile fatty acids (e.g. acetate, propionate, and butyrate) and lactic acid. The production of end-products is highly dependent on factors including pH, temperature, hydraulic retention time and the types of sugar being fermented. Results of this current study indicate that the pH and hydraulic retention time played significant roles in determining the end products from the anaerobic acidification of maltose and glucose. Under uncontrolled pH, the anaerobic acidification of maltose ceased when pH in the reactor dropped below 5 while anaerobic acidification of glucose continued and produced ethanol as the main end-product. Under controlled pH, lactic acid was found to be the dominant end-product produced from both maltose and glucose at pH 5. Acetate was the main end-product from both maltose and glucose fermented at neutral pH (6 and 7). Short hydraulic retention time (HRT) of 2 days could induce the production of ethanol from the anaerobic acidification of glucose. However, the anaerobic acidification of maltose could stop when short HRT of 2 days was applied in the reactor. This finding is significant for industrial fermentation and waste management systems, and selective production of different types of organic acids could be achieved by managing pH and HRT in the reactor. GRAPHICAL ABSTRACT

Ethanol and lactic acid production from sugar and starch wastes by anaerobic acidification

Anaerobic conversion of carbohydrates can generate various end‐products. Besides physical parameters such as pH and temperature, the types of carbohydrate being fermented influences the fermentation pattern. Under uncontrolled pH, microbial mixed cultures from activated sludge and anaerobic digester sludge anaerobically produced ethanol from glucose while producing lactic acid from starch conversion. This trend was not only observed in batch trials. Also, continuous chemostat operation of anaerobic digester sludge resulted in the reproducible predominance of ethanol fermentation from glucose solution and lactic acid production from starch. Different feeding regimes and substrate availability (shock load versus continuous feeding) in glucose fermentation under non‐controlled pH did not affect the ethanol production as the major end product. Shifts in feed composition from glucose to starch and vice versa result in an immediate change of fermentation end products formation.

Biological Methods of Odor Removal in Solid Waste Treatment Facilities

Solid waste management facilities invariably generate odors that can be a nuisance issue for workers and the community at large. Prevention and minimization of odor generation through better housekeeping is important, as well as appropriate odor removal treatment methods that are integrated and designed into the waste management facilities from the beginning. Biological odor removal methods have attracted increasing popularity because they have been proven to be cost-effective, obviate the need for chemicals, and entail the lowest impacts to the environment. In this chapter we review odor compounds likely to be generated at solid waste treatment facilities and provide an overview of established and emerging biological methods for odor control. The mechanisms of biological odor removal and factors affecting the performance of bio-treatment processes are described and discussed. A case study of a large-scale odor biofilter at a solid waste composting facility is provided. We conclude by providing a summary, current challenges, and future development potential.