Last year I blogged about a visit the Southerly Wastewater Treatment Plant which serves a population of more than 601,000 in the Greater Cleveland area. Built in 1927, Southerly is one of the largest activated sludge plants in the country. In the activated sludge process solids are separated from the liquids using a sequence of treatments to reduce the amount to water in the solids to the point where it can be incinerated. The fluid effluent is processed and then flows into the river. The ash from the incineration process goes into ash lagoons.
What really surprised me was the amount of energy required to process wastewater. The plant's electricity requirements are about 7.8 million kWh/month, primarily for pumping, which costs $492,000 per month. The natural gas requirements for the incinerators is 37,000 mcf/month per month, which costs about $417,000 per month.
Recent research using microbial reverse-electrodialysis cells (MRC) to digest organic solids to produce electricity suggests that the energy flow in treating wastewater could be reversed. It is estimated that with this process there is ~9× more energy available in domestic wastewater than is required to treat it using conventional methods.
Reverse electrodialysis (RED)
Reverse electrodialysis is a way of generating electricity from salt concentration differences. It involves pumping salt water and fresh water through alternate cells separated by ion-exchange membranes to create a differential voltage. With many cells side-by-side, RED can be used to generate electricity. Because many cells are required and the relatively high cost of ion-exchange membranes, the cost of generating electricity has been estimated to be $6.79 per kWh, which is very high compared to other sources of electric power.
Microbial fuel cell (MFC)
A microbial fuel cell converts chemical energy to electrical energy using bacteria. An MFC consists of two compartments separated by an ion-exchange membrane. Organic material is decomposed by the bacteria in one compartment producing hydrogen ions which pass through the membrane to the other compartment while electrons flow through an external electric circuit.
Microbial reverse-electrodialysis cell (MRC)
In 2011 researchers combined RED and MFC to create a new type of bioelectrochemical system for producing electrical power, called a microbial reverse-electrodialysis cell. An MRC avoids the necessity of having many RED cell-pairs. Pumping seawater and river water through an MRC consisting of only five RED cell pairs produced up to 4.3 watts per square meter (Wm-2). Pumping accounted for <2% of the produced power. These results show that the combination of an MFC and a RED stack is a new type of system that can be used to more efficiently capture energy from salinity differences between seawater and river water.
Application of MRC to wastewater
MRC applications are currently limited to coastal areas where both fresh and salt water solutions are available.
Recently it has been proposed that thermolytic solutions such as ammonium bicarbonate, which can be concentrated with low grade thermal energy, may be able to provide the salinity gradient energy source for an MRC. Capturing waste heat (>40°C) energy with ammonium bicarbonate is possible with well proven technologies that produce high concentrate and low concentrate salt solutions. The resulting energy difference between the high and low concentrate solutions is greater than that between ocean and river water.
Experimental results indicate that such a system would be capable of producing 0.94 kWh of electrical energy per kilogram of organic waste. Conventional treatment of wastewater, by contrast, typically consumes around 1.2 kWh per kilogram. It is suggested that the MRC process with a closed system of ammonium bicarbonate regeneration could substantially change the energy balance for wastewater treatment.