Reactor Applications and Process Monitoring for Improved Anaerobic Digestion
Abstract: Biogas, a mixture of methane and carbon dioxide, is produced by microorganisms upon degradation of organic matter in an anaerobic environment. It has been realised that by utilising anaerobic processes the organic pollutants in wastewater and organic solid waste from anthropogenic sources can be converted into a renewable source of energy. However, to fulfil the potential of anaerobic digestion in a sustainable society several measures must be taken. Anaerobic digestion has the potential to be applicable to waste and effluents which are currently not being utilized provided that suitable reactor concepts are developed. For example, within the pulp and paper industry, there is a potential to treat more inhibitory and less degradable effluents to realise energyproduction provided that suitable methods and processes can be found. Upflow anaerobic sludge blanket reactors (UASBs) have been used for anaerobic treatment of effluents from the pulp and paper industry. Treatment of a mixture of sulphite evaporator condensate (SEC) and caustic extraction liquor (CEL) in a UASB compared favourably to treatment in a full-scale contact-process (CP). The UASB reactor showed a higher reduction in the chemical oxygen demand (COD) (74-84%) at an organic loading rate (OLR) of 2.3 kg COD (m3 d)-1 than the CP (65% COD reduction, 1.4 kg COD (m3 d)-1). However, the CP effluent contained lower concentrations of volatile fatty acids (VFAs) than the UASB effluent. Upon treatment of spent sulphite liquor (SSL) in a UASB the maximum influent COD concentration and OLR leading to stable reactor performance were 78 g COD l-1 and 51 g COD (l d)-1, respectively. In batch reactors the biogas production was totally inhibited at COD concentrations above 40 g COD l-1. This adaptation of a granular community to increased levels of toxic substrate illustrates one advantage of the granular process when dealing with toxic effluents. Low-rate systems commonly applied today are normally loaded far below their capacity to prevent overloading and instability. The utilization of these systems can be increased by applying new monitoring systems that provide information for control so that the loading of the process can be optimised without jeopardizing the stability. A spectrophotometric method for monitoring alkalinity in anaerobic digestion processes was developed. The method is based on measurements of pH-indicators in mixtures of an acid and samples from the reactor system. Good correlation was found between the traditional titration method for alkalinity measurement and the spectrophotometric method when measuring samples from an anaerobic digester. An on-line monitoring system based on the method was designed and applied to a UASB. A pulse organic overload to the reactor lead to a decrease in the partial alkalinity, which was registered by both methods. The response of the monitoring system correlated well with the traditional titration method. A fully automated device based on the spectrophotometric method was used for on-line monitoring a full-scale anaerobic digester operating on a mixture of municipal wastewater sludge and potato-processing waste. The system was found to respond to an overload situation in which the bicarbonate concentrations in the reactor decreased. Such an on-line monitoring system is of value with regards to increasing the efficiency of certain anaerobic processes.
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