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Project shows a nose for efficiency

A study to undertake audits at wastewater sites has indicated the best way to optimise sludge handling facilities, helping to tackle odour, minimise costs and increase revenue, says Paul Lavender of Aqua Enviro

Understanding the process interactions on large WwTWs is key to minimising the risk of odour and optimising process efficiencyUnderstanding the process interactions on large WwTWs is key to minimising the risk of odour and optimising process efficiency

Water companies are under increasing pressure to continually reduce their impact on the wider environment. Specifically, wastewater treatment works (WwTWs) must:

  • Meet increasingly stringent Environment Agency discharge consent limits
  • Prevent the release of odorous substances
  • Comply with increasingly stringent carbon commitments
  • Develop innovative methods of saving and generating energy to qualify for financial incentives, such as ROCs

In order to achieve these goals it is essential to optimise the efficiency of all treatment processes. However the interactions of these processes are not always clearly understood or defined and the poor performance of one process unit can have a number of knock on effects on other processes. Likewise, what may appear to be a cost saving in one area, may be more than offset by costs incurred in other process units.

Therefore, in order to fully realise the cost savings that can be achieved, whilst ensuring consent compliance, it is necessary to take a holistic approach, incorporating both the wastewater and sludge handling facilities.
Aqua Enviro has completed site audits at a number of major treatment works in which the performance and energy efficiency of each stage of treatment has been assessed. By identifying the plant bottlenecks, corrective actions are then recommended to optimise performance. By establishing a series of key performance indicators (KPIs) for each process unit, the operator can ensure that assets continue to operate within optimal ranges.

These studies have focused on the sludge handling facilities where the greatest gains are usually achieved by streamlining operation and minimising the opportunities for sludge degradation to occur prior to thickening. This is critical to not only minimise the return liquor loads and maximise biogas production, but also to minimise the risk of odour, thus maximising the performance of the plant from both a financial and regulatory perspective.

Process interactions

The biologically active nature of sludge means that when it is stored, anaerobic conditions rapidly develop and degradation occurs, leading to the solubilisation of both organic material and nutrients. These effects can result in a significant decline in performance of treatment, loss of revenue and unnecessary cost through the following:

  • Increased aeration requirements
  • Reduced biogas yields
  • Increased odour treatment requirements

A recent audit carried out at a large municipal wastewater treatment works achieved cost savings of >£250,000 simply by changing the primary sludge tank configurations to reduce the residence time of the sludge prior to thickening. In summary the effects were:

  • Reduced aeration requirement due to improved return liquor quality
  • Reduced chemical oxygen demand (COD) solubilisation prior to sludge thickening resulting in increased biogas yield
  • Reduced chemical requirement for odour control units
  • The impacts and costs of sludge solubilisation are discussed in the following section


If solubilisation of COD occurs in the sludge during storage, a significant proportion of this solubilised COD is removed by the thickening process and returned to the activated sludge plant in the return liquors.

As this additional organic load carries a significant additional aeration requirement, reducing the solubilisation of COD could provide a saving of up to £224,000 per annum for a site treating 100t/d dry solids (DS).

Biogas yields

The repercussions of excessive sludge storage can also have significant effects on digester performance in terms of the biogas yields produced. In addition to the revenue produced from improved biogas yields, there is often the additional benefit of government incentives such as ROCs, which are currently worth around £46 per MWh.

The impact of organic solubilisation on operating costs and biogas yields can be quantified for a range of scenarios. Again, for a site treating 100t/d DS, the lost revenue from sludge solubilisation could be over £1M/year.

The impact of just 10% COD solubilisation occurring on a site treating 100t/d of DS (equivalent to 170t of COD) would be (combined income lost plus increase in treatment costs) up to £1.3M per year.

These costs do not take into account the additional costs associated with increased sulphide production in terms of biogas scrubbing and odour treatment, nor do they include polymer costs, which may increase through changes in the dewaterability of the sludge, so the actual financial implications are likely to be greater than this.

Odour control

Odour generation is becoming of increasing importance to water companies. As odour complaints can constitute a statutory nuisance, these can lead to costly disciplinary action by the local authority. For this reason, sites may require the installation of expensive odour monitoring and removal systems which require constant replenishment of costly chemical products such as caustic.

Where the installation of such systems is unavoidable, any strategy for minimising chemical usage while ensuring the release of odorous substances is kept to a minimum can be of significant financial benefit. Odours generally become a problem on many sites as a result of excessive sludge storage.

“Significant improvements to the treatment process and on energy usage can be achieved through the identification of the correct KPIs for each individual process unit”

During such times, anaerobic conditions can prevail, allowing for the metabolism of sulphur-containing compounds, producing mercaptans and hydrogen sulphide.

The generation of hydrogen sulphide is particularly problematic due to its strong odour and also its high acidity, which can damage certain process units.

At one WwTW with a population equivalent of 1M, odour issues were becoming commonplace, resulting in pressure from the local authority and the Environment Agency. A site audit was carried out to monitor redox potential, pH, soluble COD, sulphides,
volatile fatty acids and ammonia at each process stage.

Following a site audit, the area of greatest odour generation was identified as deriving from the primary sludge thickening and storage process. Following simple changes to pump set points, sludge retention times were reduced
to <1 day.

This resulted in a rapid drop in complaints from the local community and a net saving of >£250,000 per annum through reduced aeration requirements, increased biogas yields and reduced chemical costs for the odour control units.

Polymer costs

Polymer use is of significant cost (approximately £2,000/t of raw product) and the quantity required for effective dewatering is largely governed by the type and condition of the sludge. Excessive storage times can increase
the required dose as can the type of sludge being processed.

Optimisation of the primary treatment tanks results in multiple benefits in terms of the sludge ratios, as well as reducing the load to the secondary plant and ensuring maximum biogas yields are achieved.


Excessive or non-optimal sludge storage can lead to loss of revenue, an additional aeration requirement, reduced biogas yields and excessive chemical usage on odour control units. Significant improvements to the treatment process and on energy usage can be achieved through the identification of the correct KPIs for each individual process unit.

However, it is often the case that the interaction between treatment processes is overlooked. It is in these areas that great gains can be made both in terms of treatment and energy saving/generation, therefore a holistic approach is required to optimise overall asset performance.

Storage time (days)

% COD solubilised

COD load (t/d)

Oxygen demand kg.02/d

kW required at 2kg.02/kWh

Cost per day at £0.09/ kWh

Annual cost (£,000)






















Impact of sludge storage on aeration costs for a site treating 100t/day dry solids


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