Driving forces

Disposal route protection, bioenergy production and increased volatile matter conversion are the new market drivers for sludge treatment in the UK. Aidan Cumiskey and Dorian Harrison of Monsal explore some of the issues when considering investment in the new generation of technologies

Recent significant developments in advanced digestion now available, largely in response to tightening bacteriological and pathogen residuals, have now achieved a critical mass with some dominant approaches, alongside others which have more recently migrated from pilot to full scale. This critical-mass stage vitally reflects achievement of resilience in plant and product quality. While this article focuses on solutions to the new drivers, it is essential that the long-standing drivers are not prejudiced.

The key driver during the first wave of digestion plant investment post 1930s was odour reduction, to enable once remote WwTWs, now engulfed by urban sprawl, to continue to be acceptable neighbours to the new property owners and dwellers. Now in, 2006, continuing to protect the most environmentally beneficial and cost-effective route of land recycling requires yet higher levels of sludge disinfection via further reduced bacteriological and pathogenic residuals.

In order to resiliently deliver this new challenge without compromising the earlier drivers and benefits, recent sustained performances show that pre-mesophilic anaerobic digestion (MAD) process stages (biological, thermal, etc) followed by conventional MAD offer the way forward in satisfying this latest driver.

Monsal signed three major contracts this year to supply advanced anaerobic digestion (AAD) technology and pre-design for the sludge treatment centres for Southern Water and for Anglian Water, two leading UK water utilities. These advanced digestion projects are the first to be awarded in AMP4, which demonstrate the progression in this investment cycle from traditional single-stage digestion to more resilient and higher-performing multi-stage biological processes for the UK.

Without wholesale works redesign to abate raw sludge odours, MAD remains essential. Further, given the significantly higher overall VM reductions from the AAD processes and the increasing economic value of net biogas and generated power, the AAD pre-treatments will fund their own ongoing use – even without the strategic route value from higher bacteriological quality sludge. And other wider global sustainability benefits, such as greater methane conversion and consumption to reduce overall greenhouse gas impacts, are also delivered.

This future-proofing within the AAD process by recognising these innate flexibilities is vitally important as the viability of investments and returns over-extended periods becomes more focused. These same commercial pressures highlight the need for plant and output performance resilience – all investments in plants have to do “at least what it says on the box” to secure overall viability.

The strategic benefits and thus overall route and economic flexibility afforded by successful AAD processes are lower net solids for disposal, even lower-odour product sludge and greater biogas generation. These all depend, however, on the chosen pre-treatment option and the consistent feed solids level attained. These are the combined benefits which yield the future-proofing via innate self-financing of the pre-MAD process.

Economic benefits from the energy balance/biogeneration optimisation arise from both ROCS and Energy Offset Allowances. Turning from the drivers to the plant solutions to meet the enhanced (and treated) standards, they fall into essentially three basic approaches:

  • Biological with various reactor configurations (plug flow/batching)
  • Thermal within a temperature/time matrix having been established as conservatively resilient
  • A variety of pasteurisation hybrids ranging from thermal to aerobic exothermic pre-treatments

Many of the pasteurisation options, without or post-MAD, suffered failure through serious heating limitations and/or sludge re-infection. Each pre-treatment option can be designed to optimise a particular aspect of the anaerobic digestion process. In the case of biologically based processes, the emphasis is on completing the hydrolysis and acidication reactions in an optimal environment – see Figure 1. This allows the digesters to be optimised for biogas generation as well as pathogen reduction.

Returning to the current critical mass, proven to meet the quality standards, biological options such as EH, EEH from Monsal or thermal from Cambi and various alternatives such as that of Thames Water at Swindon, it is always useful to rigorously check each option offered in a comparable manner against a balanced score card such as:

  • Are they effective?
  • Are they resilient?
  • Do they reduce odours overall and what, if any, are the odour risk areas from any aspects of process failure?
  • Are their control windows suitably wide and stable for any anticipated feedstock (or other) variations?
  • Are the liquors odorous, of high strength or low treatability (hard COD) or do they in any way prejudice the liquid effluent stream of the WwTW?
  • Is the overall energy balance positive or negative, can the energy usefully (and cost effectively) be recovered?
  • What are the infrastructure requirements for each option? What are the impacts on existing or new digesters for pumping, heating, mixing, biogas management and odour management?

    * Are there are additional strategic benefits from the process option which are relevant to the particular works?

It is useful to look more closely at a few of these issues.

Digester loading

There are about 250 sludge treatment centres in the UK. Digester loading in conjunction with the pre-treatment process is one criteria often used when looking at different options. Generally loadings will fall into two bands:

  • Conventional: From1-2.8kg VS/m3d (up to 5.5% DS at 15d). The majority of all digestion plants comfortably operate in this range. Process performance, operational experience and infrastructure requirements are well developed in the UK
  • Advanced: From 2.8-5kg VS/m3d. About 15 advanced digestion facilities using biological, thermal and pasteurisation pre-treatment exist in the UK and operate in this range. High-rate digestion requires high-quality infrastructure suited to thick sludge conditions. There is only one operational plant in the UK loaded over 5kg VS/m3d

Effective AAD should provide high-rate loadings together with high biogas yield, high VS destruction and a high-quality sludge. Whereas conventional digestion operating at high rates (above 2.8kg VS/m3d) sacrifice quality and yield for throughput.

As an investor it is important to visit the references, speak with the operator and look at the full suite of operational data. While many plants are designed for high loading conditions, many will have only operated for any sustained period of time at levels much lower. It is important to assess the impact of operating at the design condition and looking at the effects on the digester and infrastructure around it. The overall energy balance is dependent on the volatile matter conversion of the process. There are differences between the various technologies and claimed volatile matter percentage (VM%) conversion data needs to be carefully looked at. It is important to look at all the available data such as VM% conversion, biogas generated and net energy produced to ensure that the mass balances stack up.

As a rule of thumb, both biological and thermal processes have demonstrated VM% reductions above 55%, depending on the digester loading and retention time. Figure 2 shows a typical advanced digestion facility at Blackburn (United Utilities) employing biological pre-treatment and currently delivering 53-57VM% conversion.

Figure 3 shows typical overall energy balances (usable and otherwise) between various pre-treatments for a 10,000TDS/annum advanced digestion facility. The type of energy required will have a big impact on choice of technology. As would be expected biologically based processes with operation at thermophilic temperatures or below will always have positive energy balances at feed DS% typically above 6%DS. Use of supplementary fuel to meet heat demands or the use of biogas for process requirements needs to be factored in the financial model when comparing different systems. The net energy available from biogas identified as electrical power generation on Figure 3 is a useful benchmark for each process.

Return liquors from thickening or dewatering operations from the advanced digestion processes need careful consideration. A full mass balance should be provided for key nutrients including COD, BOD, nitrogen and phosphorus.

Most pre-treatment flow sheets will be different and exhibit different liquor strengths. In all cases a technical and financial assessment on the return liquor impact is required. Liquor treatment technology is now well established with many options including side stream SBR/MBR treatment, ammonia stripping or combination with the main WwTW (capacity allowing) available. Generic data will be available from most technology providers, as a general rule, the more intensive the process the stronger the liquor. Capital cost, alkalinity requirements and aeration costs can be considerable for large centralised sludge treatment centres with advanced technology. There is still limited experience of full-scale design and operation of advanced digestion facilities in the UK, so caution should be the norm in this area.

The odour potential of the sludge from each process is not well documented and only indicators are available. VFA levels in the sludge are one indicator as to the odour potential of any digested product. Typically a very well stabilised sludge from a standard mesophilic digester will have residual VFA levels below 100mg/l. It is safe to assume that higher residual VFA levels will have a higher odour potential. A careful audit of existing advanced facilities should be conducted with a focus on the type, complexity and operational track record of any odour management equipment associated with the pre-treatment technology.

The anaerobic digestion process is a pivotal process in the sludge treatment flow sheet. This key point is often neglected and any engineering constraints on the overall flow sheet is dictated by the process conditions in the digester. By operating the digester with high solids, say more than 6%DS feed, all the pre- and post-infrastructure needs to be able to manage this process duty.

All the unit processes of pumping, mixing, heat transfer, biogas management and odour management need to be designed for the onerous duty of thick sludge. Water utilities should look for a track record for any component product supplied in the flow sheet. As a general rule the more highly loaded the digesters, the greater the dependent on high-quality infrastructure associated with these systems. Some of the problems experienced with highly loaded digesters include foaming and instability and careful attention should be paid to the detail.

A balanced investment across the pre-treatment technology and any necessary improvements to the digester is required. There is no point in putting in a pre-treatment technology which necessitates very thick sludge if all pre- and post-digestion infrastructure does not have a track record in this difficult duty. A financial balance should also be made between investment in pre-treatment (mostly M&E plant – typically a lower asset life) against providing some additional digestion capacity (mostly civils – a longer asset life) and the acceptable digester loading on any particular site.

Generation of volatile fatty acids (VFA)

Biologically based systems optimise both stages of hydrolysis and acidification. Both single-stage and multistage biological processes generate significantly higher levels of VFAs than normal feed to digesters. Figure 4 shows some VFA profiles generated from the Monsal EH system at Bromborough at two different temperatures.

Operational data from both single and multistage pre-treatment biological reactors shows VFAs ranges typically 5,000-15,000mg/l (acetic acid is in the range 2000-4000mg/l for these sites with the other acids proprionic, valeric, butyric also present to make up the total VFA level).

A reliable VFA source is required for WwTWs employing biological phosphorus removal processes. With biological AAD, It is possible therefore to separate a proportion of the pre-treated sludge, thicken it and provide VFA-rich centrate to the WwTW. The pre-treatment AAD technology becomes in effect a VFA fermenter.

The plant operator would therefore have a choice to use VFAs for biogas generation or to augment biological phosphorous removal depending on the particular mass balance for that site.

Thermal drying – optimisation

A process-flow sheet combining anaerobic digestion and thermal drying is now well established in the UK with a number of utilities well versed in its operation. With rising energy prices, it now also makes good commercial sense to review options for retrofitting advanced digestion processes to existing drier sites.

Combining biological AAD and high solids digestion at drier sites makes a significant difference to the energy balance. Already energy positive, the high VS% conversion of advanced technology at typically over 55% significantly improves the energy balance. With a low-temperature, pre-treatment system, the condensate from the drier can be fully utilised for the pre-treatment stage heat balance.

In summary, this overview has sought to put in context proven advanced anaerobic digestion (AAD) options as continuing matches between drivers and solutions in which the mesophilic anaerobic digestion (MAD) process has successfully delivered for over a century.

In the challenge of continuing to secure the strategically optimal route of sludge recycling to land, the MAD process has been again pushed to deliver enhanced quality sludges to virtually undetectable levels of key bacteriological indicators. Rigour in evaluating AAD options needs to ensure that all the accumulated and still-needed benefits of MAD are retained, indeed the original odour reduction driver is soon likely to regain centre stage. nnn

The authors would like to thank Colin Brade (Monsal), Prof Gerald Noone (Newcastle University/Monsal), Dr Garry Hoyland (Mott McDonald) and Dr Son Le (United Utilities) and for their valuable contributions and permission to use extracts in the preparation of this article

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