Anaerobically speaking

Aqua Enviro Technology Transfer has recently held a conference on anaerobic digestion. Here we describe some of the topics up for discussion

Because of increased interest in advanced digestion, the Advances in Technology for the Anaerobic Digestion of Municipal Sludge conference was organised by Aqua Enviro Technology Transfer on June 14 in Manchester. The conference brought together leading speakers in the field of anaerobic digestion with an audience that was focused on this subject area.

The first speaker, Prof Gerald Noone, gave a wide-ranging paper reviewing the drivers for advanced anaerobic digestion (AAD). One of the main drivers for AAD is to achieve pathogen-reduction standards. But not all pasteurisation systems give improved digestion efficiency, and not all efficient digestion systems give guaranteed pasteurisation, so it is important to understand the motivation for digester improvements.

AD has always been seen as a way of reducing odour. While many WwTWs in the UK have worked hard to reduce odour on site, many still export digested products that are odourous due to inadequate treatment. And these will affect the sustainability of biosolids recycling.

Even if the agricultural route is lost or becomes restricted, the additional volatile solids reduction and in some case better dewaterability provided by AAD means that there are less overall tonnes of cake for disposal. The mass and energy balance will be improved so that subsequent sizing of alternative routes such as drying or incineration is reduced.

By far the biggest change in recent years is the increase in energy costs experienced by the water utilities combined with the financial incentive of renewable obligation credits (ROC). The overall opportunity of avoided power costs and ROCs is now a major financial factor in decision making for biosolids projects. Professor Noone then demonstrated new AAD systems that produce significantly more biogas. Also, increasing the dry solids for digestion makes a significant difference to the power produced per cubic meter of digestion capacity. So by upgrading existing digesters to take more feed, more power can be produced.

The next speaker, Dr Mick Dawson from BHR Group gave a talk on options for enhancing digester mixing, which highlighted the importance of digester mixing and its link with biogas production, particularly with the trend to higher dry solids digestion making earlier design assumptions outdated. The bulk of his paper detailed the tests done by BHR to measure and model performance and how this can affect key design criteria; this included measurement of active volume, scale physical modelling, studying rheology and computer fluid dynamics (CFD).

Dr Dawson gave a comprehensive review of state-of-the-art mixing systems. He pointed out that there were still gaps in our understanding, including the link between mixing and process, the correlation between rheology and prediction and validating CFD in real life. Dr Dawson concluded that much that had been learnt about measuring and modelling digester performance can be applied to AAD, especially where existing digesters are being upgraded.

Prof David Stuckey of Imperial College gave a talk entitled Potential for Anaerobic Process to Treat All Wastewater. He said that the prejudice that AD is slow and only applicable to a limited spectrum of organic compounds is groundless. The problem is not biological activity but doubling time (ie the ability to retain active biomass at short treatment HRTs).

Professor Stuckey then went on to consider the following statements, all of which he considered as untrue:
  • AD can only be used for high-strength wastes
  • Reactors have to operate at 35ÚC
  • Reactors must have HRT >10 days
  • They can only remove 50-80% COD
  • They are unstable and difficult to operate
He went on to describe a novel Submerged Anaerobic Membrane Bioreactor (SAMBR). At pilot scale, the SAMBR has performed well with minimal sludge production and maximum biogas production. COD removal was more than 91% at three hours HRT.

Frank Rogalla from Black & Veatch reviewed Four Pathways to Biosolids Stabilisation. This was the subject of a previous Technically Speaking article (WWT, October, 2005) so is not reviewed here.

Son Le of United Utilities (UU) described thermophilic biological pre-treatments for mesophilic anaerobic digestion, in which he reviewed the ranges of time and temperature that are required for pasteurisation.

Lower-temperature pasteurisation in the range of thermophilic temperatures although requiring longer HRT (at least one to two days) does at least give the opportunity for some biological hydrolysis that can enhance digestion. He described the Alpha-Biotherm system at Ellesmere Port that uses a thermophilic aerobic digestion pretreatment stage. He estimated that at best the system only had a neutral effect on biogas production due to the energy lost in the aeration stage. He then went on to discuss the various combinations of thermopilic and mesophilic digestion being used in the USA and Germany and focused on temperature-phased anaerobic digestion (TPAD) that normally has a five-day thermophilic stage. TPAD systems are stated as having a VS reduction of 50-60% VS.

Finally, he gave details of the latest work being done by UU at Blackburn where a system combining enzymic hydrolysis (EH) and 55ÚC pasteurisation has been in operation since January 2006. The system was commissioned as a six-tank EH system and converted to enhanced mode on January 16 on the final three tanks to give a pathogen-free product.

Keith Panter gave a wide-ranging paper on pre-treatment in anaerobic digestion. The focus was on modern two-stage processes that have been developed to maximise biogas production and some cases to increase dewaterability. He categorised the available treatment methods and their mechanisms. He then described the mechanisms for disintegration and hydrolysis and their subsequent effect on digestion and on extra-cellular polymer (ECP) that affects dewaterability; showing that heat treatment has the largest effect on dewaterability because of increased solubulisation of ECP.

Panter gave examples of disintegration systems including ball mills, ultrasound, lysing centrifuges and high-pressure homogenisers. Results from these systems have been promising but often variable. He then described combinations of acid phase digestion (APD) and thermophilic digestion. Data from all known TPAD published data showed average VS reduction of 52% and one UK APD systems only gets 45% VS reduction - possibly due to the need for some form of disintegration.

He showed data from two plants using Cambi Thermal Hydrolysis Process (THP). In this case, the special effect on dewaterability was demonstrated in a halving of cake volume at one plant digesting only SAS. The latest performance data from Chertsey WwTW was presented and shown in Table 1.

Grzegorz Bujoczek of Black & Veatch gave a paper on foam control in digesters. The findings of the work done by BV show that maintaining consistent VS loading to digesters prevented and controlled foaming. Research showed that low-level sonication of 100% SAS controlled foaming caused by secondary sludge but at high level of disintegration that foaming was exacerbated due to higher substrate availability.

Dr William Barber of United Utilities gave a paper on the effect of AAD on mass and energy for downstream options of dewatering, drying, agricultural recycling and thermal projects. Using a rigorous approach and worked examples, he demonstrated that digestion and dewatering can reduce cake tonnage to 76% or less of original. Drying projects are more or less halved in terms of size and energy consumption. For thermal projects such as incineration, the calorific value of the sludge is reduced. But this can be offset by improved dewatering in some technologies that can lead to a positive effect on CV and a much smaller project.

Overall, the conclusion from the conference was that any medium-to large-size plant or sludge centre will benefit from AAD, both in terms of cost and quality. Mature projects in existence in the UK are clearly demonstrating this.

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