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Climate change policy development

Over the past few years, energy generation advances at treatment plants in the UK have taken place against a background of major governmental policy initiatives concerning sustainable development and carbon reduction. The Renewables Obligation, which sets ambitious targets for the UK to generate a proportion of its energy from renewable sources, has been acted upon by a number of operators in the wastewater industry, primarily through energy from biogas. However, financial incentives for generating renewable energy (Renewables Obligation Certificates are worth £45-50 / MWh), aside from reduced grid energy purchases, are offset by the associated monitoring, auditing and accreditation requirements and costs. More recently, participation in the EU Emission Trading Scheme (EUETS) necessitates similar monitoring and the employment of verifiers.

IPPC UK regulations also impacts upon energy generation and use in the wastewater industry. The IPPC's permitting process assesses the impact of emissions on the atmosphere from, for example, CHP units and also directly regulates energy use which can entail implementing efficiency improvement conditions. A further aspect of IPPC regulates water use efficiency.

The wastewater treatment industry has responded to a number of these, and other, policy drivers. Increasing volumes of wastewater require treatment to higher standards, particularly where effluent is directly discharged to the environment. There is a greater emphasis on wastewater reuse; this often needs energy intensive technology, including membrane solids exclusion or reverse osmosis technology. However, although the benefits derived from high standards of water treatment and water reuse are considered against the associated increased energy requirements by the industry, costing mechanisms are not entirely transparent. Despite the policies of both sustainability and environmental protection being in place, there is no direct mechanism to incentivise the industry to consider long term sustainability. In practice, decisions not made on purely financial terms, often over a five year payback period, with regard to wastewater quality requirements, are hard to justify and difficult to fund.

Energy efficiency improvements

Even in the absence of direct incentive mechanisms to deliver sustainability policies, WWTW operators are striving to reduce their energy use and carbon footprint by adopting a range of energy efficiency measures using a combination of innovative plant design, changes to procedures and new technology. This is, in part at least, because of the high energy prices (Figure 2).

Two GE ZeeWeed membrane bioreactor (MBR) WWTW systems from GE Water & Process Technologies (www.ge.com/water) at Pooler and at Fowler Water Reclamation Facilities (both in Georgia , USA ) illustrate effective energy efficiency measures. MBR technology is recognised worldwide for reliably producing high quality effluent, offering a more reliable performance than CAS. This comes with an associated energy cost, however. One of the main power requirements is for membrane aeration, necessary to maintain treatment performance and to reduce cleaning intervals.

Fowler WWTW produces high quality effluent suited to urban water re-use or surface water discharge at a capacity of 9500 m3/d. Substantial power savings were achieved here at the design stage, with modifications to conventional MBR designs saving 470 kW through the elimination of 30 items of equipment. These modifications involved consolidation of piping and flow pumping, and the installation of a common blower header to scour multiple membrane cassettes in a non-continuous sequential pattern. Further energy savings were achieved through procedural changes during the first 12 months of operation, which in combination saved an estimated 40% energy. One key improvement was the reduction of sludge recirculation (saving up to 30% energy for this component) during night-time under low influent conditions - the denitrification of lower nitrate loads in the anoxic zone is maintained at these times, since less oxygen is produced from the denitrification process itself. Further, up to 30% of the energy for process air blowers is saved by shutting them off for 1-3 hour periods, a measure made possible since a cascade weir entrains up to a third of the required oxygen.

At both Fowler and Pooler WWTW (a 11400 m3/d municipal MBR plant, Figure 3) GE's Intelligent Aeration Control (IAC), an advanced cyclic air scouring technology, has resulted is substantial energy savings running to an estimated 200 000 kWh at the latter site representing about 50% of the air scouring energy requirements. Utilising ZeeWeed MBR technology during periods of low influent flow, the aeration-off time period can be safely increased whilst maintaining performance. Under normal 10/10 cyclic aeration, each membrane in a train is scoured for 10 seconds before scouring is switched to a different membrane cassette for 10 seconds. IAC uses a more complex pattern of membrane scouring responsive to the current influent load. Extension of the intervals between scouring to 30 seconds (10/30 eco-aeration) is possible under non-peak influent loads.

Emission minimisation

Advanced technologies are also available to minimise emissions from WWTW. For example, the recently released N-Tox from Water Innovate (www.waterinnovate.co.uk, Figure 4), is a continuous real-time nitrification monitor for use at CAS municipal and industrial plants. The nitrification process, converting ammonia to nitrate via a nitrite intermediate, is microbially mediated, the efficiency of which is dependent upon a range of highly variable factors including hydraulic retention time and wastewater composition. N-Tox monitors nitrification efficiency in real-time through the infrared (IR) measurement of nitrous oxide (N2O) in the gas phase immediately above the effluent. N2O production rates rapidly increase as oxygen depletion and other nitrification inhibiting factors allow nitrogen reductase to convert NO2- to N2O, making N2O an excellent indicator of nitrification inhibition.

Water Innovate's automated device substantially improves on existing nitrification monitoring approaches, being non invasive and more robust and, importantly, by providing real-time results. Corrective engineering at the WWTW upon alert to nitrification inhibition can prevent elevated ammonia discharges. Since N2O gas increases can be detected even when dissolved oxygen concentrations are not yet reduced below critical values, treatment failures can be remedied that might otherwise only be detected several hours later. In the context of WWTW carbon footprint, the rapid detection of N2O and corrective abatement is desirable since nitrous oxide has 310 times the radiative forcing potential of carbon dioxide, and is a critically important greenhouse gas. Operators can more readily optimise their whole treatment system in order to minimise their greenhouse gas releases utilising advanced monitoring capabilities such as N-Tox.

Conclusions

The scope for synergy between wastewater treatment systems and energy generation is much greater than is currently employed. Biogas generated from anaerobic sludge digestion has been afforded the greatest attention by the industry, building upon existing experience in CHP systems from other sectors. The recent focus for biogas has been on understanding the process of AD sufficiently to maximise biogas generation rates. Currently widely used at large WWTWs, meeting much of the increased energy demand for large operators, new developments in plant technology, gas generation and gas contaminant clean-up will make energy production from biogas feasible at smaller plants in the future. Currently less well exploited are renewable energy generation schemes using wind, solar, wave, tide and hydropower.

For the wastewater industry there is no direct incentive mechanism to adopt systems which meet the needs for sustainable development and energy use; current drivers emphasise treatment quality, and financial payback times are considered over much shorter time-scales. However, current high energy prices have forced operators to adopt procedural and technological measures designed to improve energy efficiency and reduce harmful emissions. Through careful plant design and by treatment processes being more responsive to variable influent loadings, WWTW can significantly reduce their carbon footprint whilst meeting the demands for high quality wastewater treatment.

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