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Veolia explains how many industrial wastewater streams have the potential to provide renewable energy.

A variety of industries from brewing and food processing to organic chemicals and papermaking produce wastewaters with high Chemical Oxygen Demand (COD).

To prevent possible damage to the environment and high discharge costs, these wastewaters are frequently treated prior to discharge to the river or sewer.

The process of choice for well over half a century has been aerobic biological oxidation using activated sludge.

In this process a bacterial biomass, in the presence of oxygen, oxidises organic compounds to carbon dioxide and water.

The oxygen is provided by aeration either using surface aerators that generate high turbulence or by bubbling air into the wastewater under pressure.

Either way, the process uses a significant amount of energy – typically about 1kWh/kgCOD.

So a brewery producing 500m3/day of wastewater with COD 4,000mg/l will consume about 2MWh of electricity each day just to treat that waste stream.

But what if, instead of using electrical energy to treat the wastewater, it actually supplied you with energy? That is exactly what anaerobic treatment does.

Anaerobic treatment is a biological process that operates in the absence of oxygen and the friendly bacteria convert organic chemicals into biogas, which can be used as a fuel for boilers or for green electricity generation in a Combined Heat and Power (CHP) plant.

A typical anaerobic wastewater treatment plant converts about 85 per cent of the influent COD into biogas at a rate of about 0.3Nm3 of biogas per kg of COD.

The biogas composition is usually about 70-85 per cent methane, 15-30 per cent carbon dioxide with trace amounts of hydrogen sulphide and it has a calorific value around 30MJ/m3.

So our brewery wastewater, instead of consuming 2MWh of electricity per day, could potentially generate 2,000kWh in a CHP plant.

One of the first commercial anaerobic wastewater treatment processes was the Biothane Upflow Anaerobic Sludge Blanket (UASB) reactor at a yeast production facility in Delft in 1972.

Veolia Water Solutions and Technologies acquired Biothane in 2008 and has continued to develop the process and the latest generation of high-rate Expanded Granular Sludge Bed (EGSB) plants have a footprint less than a tenth of that needed by conventional aerobic activated sludge systems.

The anaerobic bacteria naturally grow together to form dense granules about 3mm in diameter.

Wastewater enters the reactor via a distributor at its base and flows upwards through the granular sludge.

A three-phase separator at the top of the reactor allows sludge to fall back into the reaction zone, while liquid passes out through a collector pipe and biogas collects in the space above the liquid.

Conventional aerobic activated sludge processes produce a considerable quantity of excess biomass (sludge) for disposal – an increasingly difficult and expensive procedure.

By comparison, anaerobic systems generate hardly any surplus sludge and that can often be sold as a seed source for quick start-up of new anaerobic plants.

Another anaerobic process, sludge digestion, can be used to treat the sludge produced in a conventional aerobic activated sludge plant.

Typically, activated sludge plants generate about 0.5kg of biomass per kg of COD removed, so treating the brewery wastewater by activated sludge would generate a sludge containing something in excess of 1.5 tons of dry solids each day.

Waste-activated sludge is usually tankered off site to landfill – an expensive and environmentally unsustainable disposal route.

Anaerobic sludge digestion changes all that.

In the absence of oxygen in an anaerobic digester, the dying and dead cells in the sludge biomass effectively ‘turn cannibal’ and destroy themselves in a process called autolysis.

The resulting breakdown products are converted into biogas by the same anaerobic bacteria as are used in the Veolia process.

This reduces the sludge volume for ultimate disposal by up to 50 per cent and generates about 0.5m3 of biogas for each kilogram of solids destroyed.

On this basis, anaerobic digestion would reduce, for example, a typical brewery’s sludge disposal to about 0.75tons of dry solids per day and would produce some 350m3 of biogas per day with a potential for generating about 1.5MWh of electricity.

The digested sludge can be readily dewatered, leaving a fibrous digestate and nutrient-rich liquor, which can be used as a liquid fertiliser.

The environmental benefits of sludge digestion are obvious, but it also attracts support under the Renewables Obligation, which the UK government introduced in 2002.

This is the main support scheme for renewable-electricity projects in the UK and places an obligation on UK suppliers of electricity to source an increasing proportion of their electricity from renewable sources.

The current target is that 15.4 per cent of electricity should be from renewable sources by 2015.

Suppliers meet their obligations by presenting sufficient Renewables Obligation Certificates (ROCs).

These are green certificates issued to an accredited generator for eligible renewable-electricity generated within the UK and supplied to customers within the UK by a licensed electricity supplier.

Normally, one ROC is issued for each megawatt hour (MWh) of eligible renewable output generated.

Where suppliers do not have sufficient ROCs to meet their obligations, they must pay an equivalent amount into a fund, the proceeds of which are paid back on a pro-rated basis to those suppliers that have presented ROCs.

The government intends that suppliers will be subject to a renewables obligation until 31 March 2027.

In order to incentivise sludge digestion, burning biogas or digestate as a fuel for green electricity generation currently earns two ROCs for every MWh of green energy generated by this process.

As an example, a food-manufacturing site generating 20,000kg of TCOD/day in the form of wastewater could produce in excess of 6,500m3/d of biogas through anaerobic treatment, which, if used in a CHP, would be equivalent to more than half a million pounds a year in the form of ROCs alone.

Using a combination of anaerobic and aerobic biological treatment technologies, it is possible to recover most of the energy content of wastewater streams as biogas, while treating the aqueous waste to a quality suitable for discharge to surface water courses, saving money in sewer discharge costs.

A little further investment in membrane technologies can treat that discharge to potable water quality or even better, so that it can be recycled within the factory for use in washdown, process, cooling tower make-up or even boiler feed.

This reduces the need to import mains water, resulting in further cost savings.

Industrial companies these days face many challenges.

All industrial production plants need to maintain operational efficiency to remain competitive, reduce manufacturing costs, reduce energy consumption and comply with government legislation.

Reducing the carbon footprint is no longer a matter of improving corporate image – it is now a legal obligation.

But there are opportunities for companies to benefit from this move by recovering energy from wastewater and recycling the treated product.

Anaerobic wastewater treatment and sludge digestion can help industry to achieve environmental compliance, make bottom line savings and reduce carbon footprint.

Veolia Water Solutions & Technologies

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