A wastewater opportunity

A recent study¹ found that the energy contained in a sample of UK domestic wastewater was 7.6kJ/L. With over 11bn litres of wastewater being treated every day in the UK, including human toilet waste, sink and bathroom effluent and rainwater entering drains, the annual energy from domestic wastewater alone is 30 terajoules (TJ) – more than 5m barrels of oil equivalent.

When agricultural and industrial wastewater is considered, the size of this resource is even more substantial. The wastewater or effluent from the UK’s 1.85m cows, for example, is a further 5TJ alone. If we found a new oil field of this size, we would not ignore it because it smelt bad, let alone spend energy trying to get rid of it.

However, the process of extracting energy from wastewater is not simple. Wastewater is predominantly water and the energy costs of dewatering would be vast, ruling out traditional combustion processes. It is also highly complex and variable in composition, changing not only from one place to another, but also from one hour to the next. Current methods of treatment are biological and future methods of energy extraction will need to be biological too, harnessing microbes to perform work that would not be viable mechanically.

The basic principle of microbial digestion involves the metabolism of organic compounds into smaller compounds, ions such as H+, and electrons. Energy is released on bond breakage and by donation of electrons to an electron acceptor, a process called oxidation. Different electron acceptors yield different amounts of energy and bacteria that can use the available electron acceptor will dominate the system. Currently, in western countries, most large scale wastewater plants use the activated sludge process, in which a plentiful supply of oxygen, the highest energy yielding electron acceptor, is bubbled through the sewage causing bacteria to grow and multiply – feeding on and thereby removing wastewater organics in the process. The water is then released safely to the environment, and the residual bacterial sludge is removed and de-watered; in Europe this is then mainly composted or spread on fields, some goes to landfill or to be incinerated.

This process has worked well for over 100 years, safely treating the wastewater, however, the energy cost of oxygenation is huge. Other processes that can, and are, being used include trickling filters where the wastewater trickles over beds of granular material with bacteria growing on them; here oxygen from the air is used to aid digestion of organic matter. Waste stabilisation ponds can also be used, harnessing the natural ecosystems of a series of ponds to treat wastewater. These technologies are far less energy intensive but require more land and are often not viable in urban areas.

So what are the alternatives? The most tried and tested of these is anaerobic digestion, whereby anaerobic bacteria can survive without oxygen and consume the organic matter. Without oxygen, they must use the organic matter for both respiration and food. In the process of digestion methane is produced: a combustible biogas. This technology has proven its worth with high strength industrial wastewaters and with domestic wastewater in tropical countries, with many successful full-scale operations. UK water companies are increasingly investing in anaerobic technology to recover energy from the large quantities of sludge produced from the standard activated sludge process. Northumbria Water currently produces 4.7MW of electricity from a single anaerobic digester plant and has plans to open several more.

During anaerobic digestion, long chain organic compounds are broken down into smaller ones by specific bacteria, eventually a further group of microorganisms, known as methanogens, are then able to convert these products into methane and carbon dioxide. The overall chemical process can be simplified and written, using glucose as an example, as C₆H₁₂O₆ → 3CO₂ + 3CH₄.

Using basic thermochemistry equations and the tabulated free energy values of the compounds in this equation, the amount of energy released during the reaction can be calculated as 427kJ, as shown in Figure 1. The comparable calculation can be made for the activated sludge process where oxygen is involved; here the energy release is 2880kJ. This represents the energy available to the bacteria, which is clearly higher in the activated sludge process, allowing the bacteria to thrive consuming the organics and to multiply, creating the large volumes of sludge (dead bacteria cells), which need to be dealt with. With the anaerobic system, there is less energy available. The bacteria are able to live and grow, feeding on and removing the organic matter, but they multiply less, creating less sludge.

However, with the process shown in Figure 1, the same amount of energy – stored in the bonds of glucose – enters each system, and as this energy cannot be destroyed, it means in the anaerobic system there must be energy going somewhere else, for example, in the methane. This energy is stored in the methane gas and released later when it is combusted. The energy released in the two different systems is exactly the same, except with the anaerobic system that energy is shared between the bacteria and the operators who may extract the gas and use it.

This makes the anaerobic process more efficient, but also less robust, particularly when energy is further limited by colder temperatures and low strength wastewaters, such as domestic wastewater. For full scale application in domestic wastewater treatment, all we need is to find organisms – from the estimated 10³⁰ in the environment – that are resilient towards lower European or UK temperatures. At Newcastle University, for example, working closely with Northumbria Water, we are looking into the possibility of using cold-adapted microflora from methanogenic low temperature environments.

Another option is the use of microbial fuel cells that produce electricity directly from organic compounds in a battery-like cell. Here again the bacteria are starved of oxygen, but can donate an electron to an electrode; this electron passes in a circuit to a second electrode where it can be oxidised, giving off H₂O and generating current. These technologies have been developing rapidly over the last 10 years and shown to be both robust and efficient when dealing with simple organic compounds, and possibly with complex real wastes. Improvements in design, architecture and materials are ongoing, with some large-scale experiments under way.

However, none of these treatment methods will be 100% efficient in terms of the energy measured through bomb calorimetry.¹ The available energy in wastewater is typically measured as chemical oxygen demand (COD); a strong chemical reducing agent oxidises all the available organic compounds and so indicates the maximum amount of material available for bacterial oxidation. The study found that in some waste streams the energy content as measured with bomb calorimetry is significantly different from the amount implied by a straightforward conversion of COD into energy as shown in Figure 1, where the two different approaches used for formate (CH₂O₂) give different energy yields. This has implications for the choice of anaerobic systems, as some wastewater streams may be better suited to methane formation, others to fuel cell technology.

As well as the scientific hurdles, there are political and economic ones. In the UK and many other European countries, the infrastructure already in place is set up for the activated sludge process. The high capital costs needed to change this, and the uncertainty of using a different technology, coupled with the UK’s high regulation of both effluent quality and pricing structures, are obstacles to change.

In other countries such as Brazil, by contrast, anaerobic digestion is widely used to treat waste and sometimes provide energy. Throughout India, meanwhile, small scale anaerobic digestors are used at a household level, using both wastewater and food scraps, producing a biogas to fire the kitchen stove. With the ability to provide combined sanitation and energy to communities that currently may have neither, it is not surprising that the UN Development Committee has highlighted anaerobic digestion as one of the most useful sources of energy supply for future development. However, the need for sustainable solutions to energy supply and wastewater treatment is a global one, and developed countries can no longer afford to waste resources. With energy prices soaring and pushing up the cost of costs of water treatment, how long can western wastewater treatment practices survive? In the UK, if the current trends of 4% annual increases of energy use by the water sector, and electricity prices doubling in the past five years continue, in 10 years’ time the industry will face an energy bill vastly higher than the current 9012GWh. With infrastructure requiring long term planning and capital investment it is hard to see how, without drastic action, the necessary changes can be made.

The UK water industry is making some headway. In 2009-2010, it produced 665GWh of energy, 7% of its energy consumption.² Anaerobic digestion is being successfully used in the UK for some highly concentrated wastewaters from agriculture and industry, and to digest the sludge produced from wastewater treatment. Significant amounts of money are also being used to research other sustainable treatment technologies.

Progress towards sustainability requires both more money and, arguably, more acceptance of failure in the pursuit of a lasting solution. But the biggest leap we need to make as a society is to view our waste as a valuable resource, not an unpleasant problem.