Towards energy positive wastewater treatment plants

Highlights

• A novel, energy positive wastewater treatment process is presented.

• The process is based on filtration/biofiltration and encapsulated denitrification.

• Electric energy is produced though biosolids gasification followed by co-generation.

• The novel process requires 15% of the energy consumed by activated sludge.

• The process (including gasification) produces 0.11 kWh/m³ of inlet wastewater.

Abstract

Energy requirement for wastewater treatment is of major concern, lately. This is not only due to the increasing cost of electrical energy, but also due to the effects to the carbon footprint of the treatment process. Conventional activated sludge process for municipal wastewater treatment may consume up to 60% of the total plant power requirements for the aeration of the biological tank. One way to deal with high energy demand is by eliminating aeration needs, as possible. The proposed process is based on enhanced primary solids removal, based on advanced microsieving and filtration processes, by using a proprietary rotating fabric belt MicroScreen (pore size: 100–300 μm) followed by a proprietary Continuous Backwash Upflow Media Filter or cloth media filter. About 80–90% reduction in TSS and 60–70% reduction in BOD5 has been achieved by treating raw municipal wastewater with the above process. Then the partially treated wastewater is fed to a combination low height trickling filters, combined with encapsulated denitrification, for the removal of the remaining BOD and nitrogen. The biosolids produced by the microsieve and the filtration backwash concentrate are fed to an auger press and are dewatered to about 55% solids. The biosolids are then partially thermally dried (to about 80% solids) and conveyed to a gasifier, for the co-production of thermal (which is partly used for biosolids drying) and electrical energy, through syngas combustion in a co-generation engine. Alternatively, biosolids may undergo anaerobic digestion for the production of biogas and then electric energy. The energy requirements for complete wastewater treatment, per volume of inlet raw wastewater, have been calculated to 0.057 kWh/m³, (or 0.087 kWh/m³, if UV disinfection has been selected), which is about 85% below the electric energy needs of conventional activated sludge process. The potential for net electric energy production through gasification/co-generation, per volume of inlet raw wastewater, has been calculated to 0.172 kWh/m³. It is thus obvious, that the proposed process can operate on an electric energy autonomous basis.

Introduction

It has been estimated that over 20% of the total energy consumption for public utilities by the municipalities is for the operation of wastewater treatment plants (Means, 2004). Conventional treatment of municipal wastewater is indeed an energy intensive process, primarily, due to the need for supplying large quantities of air into the biological process (Shi, 2011, Svedal and Kroiss, 2011). It has been estimated that, approximately 30–35% of total cost of service in North American wastewater treatment facilities is for electric energy (WERF, 2010a). With the increasing cost of electricity, it is wise to explore alternative wastewater treatment process with low energy requirement, and further to explore the potential to utilize the chemical energy content of wastewater for power production. An additional benefit, apart from the reduced energy consumption, will be the reduction of carbon footprint (Tchobanoglous et al., 2009).

Activated sludge process is the norm for municipal wastewater treatment, especially for centralized facilities. However, activated sludge process is energy intensive, as it requires large quantities of air (far beyond the stoichiometric amount) to be pumped into the biological tank. In addition, primary and secondary sludge processing requires the utilization of significant amount of electric energy for sludge pumping, processing and disposal (Tchobanoglous et al., 2003). In middle and large wastewater treatment facilities, anaerobic digestion of primary and secondary sludge is used for partial conversion (up to 60%) of organic carbon into methane and carbon dioxide (Tchobanoglous et al., 2003). The produced anaerobically digested sludge is partially stable; however, further processing is required for safe disposal. Thermal processes have also been exploited for biosolids to energy. However, combustion can be energy positive only if the water content of sludge is below about 30%, as in the opposite case more energy is required for incineration than it is produced through combustion (WEF, 1992). On the other hand, gasification appears advantageous against incineration, and thus it is a preferable process, despite the higher degree of process complexity (Fericelli, 2011). Energy-wise, gasification has been found to have a higher potential for net electric energy production, compared to anaerobic digestion (Gikas, 2014).

Relatively small reduction in energy requirements may be achieved lately by selecting more efficient aerators, by employing more efficient aeration control processes or by improving the sludge management processes (WERF, 2009, Muller et al., 2006). Recently, an increasing number of publications has focused on low energy requirements or even on self sustainable wastewater treatment processes (McCarty et al., 2011, Gude, 2015, Chae and Kang, 2013, Nowak et al., 2011, Wett et al., 2007), most of which involve either complicated technologies, either involve the use of external renewable sources, either the proposed solution involves high capex or opex costs. However, to significantly reduce the energy requirements, the wastewater treatment concept should be viewed from a fundamentally different angle. For example, the role of energy-intensive biological process should be marginalized, while the role of energy-efficient physicochemical processes should be utilized to a maximum, where applicable. If the above is combined with the maximization of the utilization of the chemical energy contained in the wastewater, then an energy positive wastewater treatment process can be possible. It is important though, that the proposed process should have comparable or lower cost with the one of activated sludge system.

The present manuscript presents a novel, energy efficient, wastewater treatment process, with significantly reduced energy requirements (compared to conventional activated sludge process). The process consists of a combination of physicochemical processes for upfront solids removal, along with downstream low energy requirement biological filtration processes for complete wastewater treatment. Moreover, the manuscript assesses the use of the produced biosolids for electric energy production, either by anaerobic digestion or by gasification. Finally, mass and energy balances are employed to show that a positive energy wastewater treatment plant can be possible. The various sub-processes presented in the present manuscript have been investigated in pilot facilities.