**5. BES for nutrients recovery from waste streams**

and H2

into CH4

132 Energy Systems and Environment

feed with contaminants (CO2

ing ethanol from CO2

nol in a CO2

in the feed. About 100% CO<sup>2</sup>

a proof-of-concept technology that converts CO2

systems showing low energy efficiency so far [44].

Although ethanol can be produced directly from CO2

slightly reduced organic compounds can be produced from CO2

requires high temperature and pressure [40], which usually implies signifi-

) for environmental remediation [41]. Later, we review some of

into valuable chemicals and fuels within a

by a wide variety of microor-

, its production from acetate is thermody-

cant energy requirements. Here, BES can provide a suitable alternative through a bioprocess known as microbial electrosynthesis (MES), which requires much milder conditions. MES is

BES. MES meets the requirements of green chemical technologies as it uses microorganisms as inexpensive and sustainable catalysts, can be operated at ambient conditions, and can be

the target products that can be obtained through MES, paying special attention to alcohols and methane, as these molecules can be easily used as energy carriers in many applications.

*Volatile fatty acids* (*VFA*). VFA are the most studied group of chemicals synthesized in MES These

ganisms following different metabolic pathways. Homoacetogens in particular are responsible for acetate production, and recent studies have reported acetic acid production rates above 0.78 g·L−1·d−1, product titers of up to 13.5 g·L−1, and current-to-acetate conversion efficiencies of 99% [42, 43]. Butyric acid is the second most reported VFA on MES systems. This C4 VFA has mainly been observed as a co-product in acetic acid production systems, although some studies also target this product as main objective [44]. Among the later, production rates of up to 0.16 g·L−1·d−1 and maximum titers of 5.5 g·L−1 at relatively low conversion efficiencies (40%) have been achieved [45]. Other VFAs such as propionic, isobutyric, or medium chain fatty acids have been found as by-products in lower concentrations in acetic/butyric acid-producing MES

*Alcohols*. Bioethanol is a renewable fuel subjected to strong controversy as its production is related to deforestation and to the rising of food prices. MES offers the opportunity of obtain-

namically and energetically more favorable, providing that the undissociated acid exists in a slightly acidic medium [46]. Still, ethanol production through MES is far from becoming a feasible process yet as titers and efficiencies are relatively low (up to 0.5 g·L−1 and 55%, respectively) [46]. Butanol can also be produced in MES following a similar pattern as in ethanol (butanol begins to appear in butyric acid-producing MES when sufficient reducing agents are present). Butanol has been found as a by-product but not targeted as main product until date, and therefore, the low rates and efficiencies are not representative of its real future potential yet [44]. Other alcohols have been obtained through MES. Arends et al. [47] firstly produced isopropa-

reported the possibility of producing glycerol when succinate was present together with CO2

*Methane. Bioelectrochemical power-to-gas.* Methane is a valuable energy carrier and a fuel with low environmental impact compared to other fossil fuels. It can be produced from CO2 through MES by taking advantage of the ability of some microorganisms (e.g., methanogenic archaea) of using a solid surface (cathode) as electron donor. First published experiences reported production rates of 4.5 LCH4·d−1·m−2 and efficiencies up to 80% [49]. More recent studies have succeeded in improving the overall efficiency reaching maximum production




Plant macronutrients—mainly nitrogen, phosphorus, and potassium—are indispensable elements for the growth of every living beings. However, when supplied to the soil as chemical fertilizers, they can bring about serious environmental issues (e.g., eutrophication of water bodies and greenhouse gases emissions). World population growth and the ever-increasing demand for agricultural products urge the need for an adequate use of fertilizers to avoid not only their undesired environmental impact but also an eventual depletion of the limited mineral deposits, especially phosphate rock [53]. Furthermore, fertilizers production consumes significant amounts of energy, which brings additional environmental and economic concerns. For instance, 3% of total natural gas production in United States is diverted to the fertilizers industry [12], and fertilizers application in China absorbs 4.4% of China's total primary energy use [13].

Nutrients recovery from wastes can prove to be a feasible strategy to tackle both environmental and energy issues simultaneously. On the one hand, it allows to limit the amount of nitrogen and phosphorus discharged into the environment, and on the other hand, it may help to reduce the energy intensity in fertilizers production. There are several technologies available for nutrients recovery from organic wastes, among which struvite precipitation occupies a preeminent position [54]. BES can also offer the possibility of recovering nutrients from waste streams, and thanks to their ability for harnessing the bioenergy present in the organic matter, they also help to offset the energy usage [55]. Most of the research studies in the literature are mainly focused on the use of nitrogen and phosphorus recovery [56], as these two nutrients are usually found in many organic wastes.

For nitrogen in particular, this element can be concentrated (usually as ammonium) on the catholyte of the BES by migration and diffusion from the anode side. Due to the high pH of the catholyte, ammonium turns into ammonia gas which can be subsequently stripped from the off-gas [57]. The use of BES for nitrogen recovery has been explored using different waste streams, such as swine wastewater, landfill leachate or urine, and different reactor configurations offering encouraging results [58, 59]. Zamora et al. [60] demonstrated that electrical energy required in a pilot scale BES for ammonia recovery is 1.4 kWh·kg<sup>N</sup> −1, which is lower than other electrochemical nitrogen recovery technologies (for instance, 13 kWh·kgN −1 is needed to recover nitrogen for digestate using a conventional electrochemical cell [61]). Moreover, some studies have even reported a positive energy balance producing a surplus of 0.96 kWh·kg<sup>N</sup> −1 [62].

The logistical problems of setting up a pilot scale project may individually seem like minor and highly surmountable problems, but can combine to have significant detriment to both the financial cost of the project and its outcomes. These problems, and the compromises that need to be made to overcome them, can have significant and long-lasting impacts into the study being undertaken. For example, a reactor planned for startup in the summer months can be delayed into the winter months which in the United Kingdom is a significant decrease in operating temperature, and would result in very slow microbial growth, and potentially a less active and effective biofilm forming. Without a replica reactor started in the summer months,

Bioelectrochemical Systems for Energy Valorization of Waste Streams

http://dx.doi.org/10.5772/intechopen.74039

135

Financial problems can be broadly split into two main areas, finding funding to do this research and then using this funding to build practical systems. BES technologies and the complex microbiology they rely on mean that they do not fit easily into the standard technology readiness levels often used to identify different funding sources. Many of the fundamental elements of BES operate differently at different scales. There is therefore a need to do basic and fundamental science (TR level 1) such as sequencing and understanding microbial dynamics on reactors that are prototypes in an operational environment (TR level 7). Secondly, once funding is in place, different materials need to be sourced which are affordable to use at large scale but will still function in the BES. Thankfully low cost alternatives to most of the materials have now been found, with stainless steel replacing platinum cathodes [66] and cheap battery separators replacing ion exchange membranes [67]. In 2008, these two components were 85% of the costs [20]; however, in recent pilots, they account for less than 2% [67]. The carbon anode material at approximately 100 £·m−2 is now the greatest material cost. Furthermore, developing cheaper alternatives to ancillary equipments such as sensors and potentiostats, which are often expen-

we are unlikely to know the full impact of this on overall reactor performance.

sive and not designed to be robust enough for field applications is another challenge.

The first large-scale experience with BES was an MFC build and operated by the Advanced Water Management Center at the University of Queensland [68]. The MFC consisted of 12 units

of this plant, although it is known that power production was limited by the low conductivity of the wastewater and biomass proliferation on the cathode [69]. In a much recent work based on a 200 L modular MFC operated in field conditions in a municipal wastewater treatment plant, Ge and He [70] reported more than 75% COD removal rates, accompanied by a power production of 200 mW, which was enough to power part of the ancillary equipment required to operate the plant. Much more complex substrates than urban wastewater have also been used as a feedstock for pilot MFC. In [71], it is reported that a 115 L MFC was able to remove almost all of the biodegradable fraction from swine manure producing about 200–400 mW of power. Moreover, the plant, which consisted of six MFC units, also allowed to remove about

The first large-scale MEC had a working volume of 1000 L, was a continuous flow, and ran a hydraulic retention time (HRT) of 1 day [66]. The system ran for a period 100 days, heated to 31°C, with an average influent of 760 ± 50 mgCOD·L−1 (soluble chemical oxygen demand)

. There is scant information about the performance

**6.2. A brief summary of the pilot scale studies undertaken in the area of BES**

with a total volume of approximately 1 m<sup>3</sup>

50% of the nitrogen initially present in the manure.

BES also represents an ideal technology to precipitate phosphorus, together with ammonium, in the form of struvite thanks to the relatively high pH in the catholyte as mentioned before. Furthermore, BES can be used to mobilize orthophosphate from the iron phosphate contained in digested sewage sludge [63]. Cusick et al. [64] reported a P precipitation efficiency of 85% with an associated energy consumption of 6.5 kWh·kgP −1, which was significantly less than that needed by other struvite formation methods based on pH adjustment.

Therefore, the main advantage of using BES for nutrients recovery, compared to other technologies, is that they allow to limit the energy requirements by exploiting the energy content of the organic matter present in a waste [55]. Finally, although the first experiences with pilot plants have already been carried out and give hope to the development of this technology, the use of BES for nutrients recovery still needs optimization of operational parameters [65].
