WASTEWATER

Domestic Wastewater Treatment



Domestic Wastewater Treatment

Introduction

Conventional biological wastewater treatment processes, such as activated sludge, are energy demanding processes. Energy use for wastewater aeration can account for up to 50% of operating costs, with typical requirements of 500 Wh/m3, or 1 kWh for oxidation of 1 kg organic compounds removed during treatment (Rabaey and Verstraete, 2005). Aerobic treatment processes also produce large amounts of residual solids which are costly to treat and dispose ([Ahn and Choi, 2004], [Murray et al., 2008], [Speece, 1996] and [Wei et al., 2003]). The high energy requirements of these processes makes it important to investigate methods to reduce operational costs through process optimization or the use of more energy efficient anaerobic processes ([Brischke et al., 2005], [Cooper et al., 2007] and [Willis et al., 2007]).

A microbial fuel cell (MFC) is a new bio-electrochemical process that produces electricity from the anaerobic oxidation of biodegradable organic substrates. Microbes in the anodic compartment produce electrons and protons from the oxidation of organic matter, with CO2 and biomass as final products. There has been great interest in using MFCs for wastewater treatment (Habermann and Pommer, 1991), and power generation has been shown using a variety of wastewaters including both domestic and industrial wastewaters ([Aelterman et al., 2006], [Ghangrekar and Shinde, 2007], [Ghangrekar and Shinde, 2008], [Liu et al., 2004], [Min and Logan, 2004] and [Rodrigo et al., 2007]).

A higher power density of 464 mW/m2 (15.5 W/m3), with total COD removal of 40-50% and coulombic efficiencies of 20%, was achieved by flowing the wastewater through the carbon cloth anode (Cheng et al., 2006). While this flow scheme improved power output, such an approach would not be sustainable method as a practical method of treatment as the anode would foul over time. Temperature effects have only been considered in a few studies using synthetic defined substrates ([Jadhava and Ghangrekar, 2009] and [Moon et al., 2006]). All other cases treating real wastewaters were conducted under mesophilic conditions (30 °C), and have not examined effects of temperature on performance.

Nitrogen removal is another important aspect of wastewater treatment (Ahn, 2006). The nitrogen removals in MFCs may include assimilatory nitrogen uptake, dissimilatory nitrate reduction, as well as physicochemical factors (such as ammonia volatilization at the cathode) that are increased in proportion to current generation (Kim et al., 2008).

An additional advantage of using MFCs for wastewater treatment is the potential for reduced solids production compared to aerobic processes (Logan, 2008). Aerobic growth yields are typically 0.4 g COD cell/g COD substrate, with yields of one tenth of this value for anaerobic growth (Speece, 1996). There is relatively little information on solids production in MFCs, and all tests have so far examined only single substrates. Cell yields in recent MFC tests were reported to vary widely from 0.07 to 0.22 g COD cell/g COD for glucose (Rabaey et al., 2003), and 0.24-0.31 g COD cell/g COD for acetate (Freguia et al., 2007). There are no data available on solids production in MFCs using domestic ...