Maximizing sustainability by energy recovery in anaerobic treatment systems

Abstract

Traditionally, wastewater treatment systems require considerable energy input, but this has changed since modern anaerobic treatment systems were developed in the second half of the last century. The production of biogas opens the possibility to produce electric power, which may make these systems independent of external energy and may even become net energy producers. A second source of energy production in wastewater treatment systems is sludge production. After drying the sludge its combustion can generate energy, though less than the energy potential from biogas, not only due to the reduced sludge mass, but also because solid fuel has a lower conversion efficiency than gaseous fuel. It has been demonstrated that in an efficient anaerobic treatment system operating under favourable conditions (warm climate, low-sulphate concentration in the influent) ∼1/6 of the influent organic material is converted into sludge whereas 4/6 is converted into biogas and 1/6 leaves the anaerobic treatment system with the treated effluent. Normally, the production of electric power from the biogas is more than sufficient to run the treatment system and an external source energy is not required. However, this does not mean that energy production is always a sound economic solution. Economic feasibility depends on the price of electric power and generally requires the operation of a large treatment system. In this chapter, equations are derived to estimate the composition of biogas and sludge in anaerobic wastewater treatment systems and specifically the influence of sulphate in the influent is discussed. Moreover, the processes for collection and treatment of biogas and sludge are presented. The potential for useful energy generation from anaerobic treatment systems is analysed and several examples are presented in order to highlight the feasibility of the energy recovery under different operating conditions.