Some like it hot

Adapting temperature-phased anaerobic digestion is being seen as a way to upgrade plant to achieve a Class A sludge product. Frank Rogalla reports

In many countries, more than half the sewage sludge generated annually is recycled back into the soil. In the United States, regulations by the Environmental Protection Agency (EPA) established management practices for land application of sewage sludge, concentration limits loading rates for chemicals, and treatment and use requirements designed to control and reduce pathogens and attraction of disease vectors (US EPA, 1994: 40 CFR Part 503).

Public confidence
The pathogen standards are technology-based requirements aimed at reducing the presence of pathogens and potential exposures to them. Anecdotal allegations of diseases from land application of biosolids have raised doubts about the efficacy of the EPA regulations in protecting public health. In response, a review of the regulations concluded that there is no documented evidence to prove that rules have failed to
protect public health. Yet, it seems evident that increased public scrutiny and growing resistance could end land application of biosolids which have not undergone treatment for pathogen removal.

Rather than relying on sludge treatment and its exposure to air and sunlight to minimise the harm of bacterial pathogens and viruses, treatment facilities are required to actively reduce pathogens in order to meet Class A standards. The requirements for Class A pathogen reduction consist of two parts, performance and operational, both of which must be met:
  • Performance compliance - reduce the densities of faecal coliform to less than 1,000MPN/g/TS, or salmonella to less than 3MPN/4g/TS. These were the 'detection limits' for faecal coliform and salmonella at the time of rule promulgation in 1994. The intent is that Class A biosolids be essentially pathogen-free, minimising the risk of infectious disease transmission through casual contact or ingestion.
  • Operational compliance - ensure that 'every particle' has been exposed to conditions known to be effective in rendering the biosolids essentially 'pathogen-free'.

    The emerging body of research findings on advanced digestion systems from prominent utilities and research organisations identifies how anaerobic digestion systems have to be designed and operated to achieve Class A compliance. This has propelled wide-
    spread interest and experimentation in the US in upgrading the performance of
    conventional anaerobic digestion processes to meet these criteria.

    Regulatory requirements
    Proper design and operation of any one stage of digestion at thermophilic temperatures guarantees performance compliance, but it is also obligatory to ensure sufficient retention time at thermophilic temperatures to minimise the probability of pathogen short-circuiting. Many of the advanced digestion schemes - such as thermophilic digestion, temperature-phased anaerobic digestion (TPAD), and three-phase digestion - operate in the continuous or semi-continuous flow modes, which fail to satisfy the requirements for
    Class A. For these systems, there are two routes for complying with the operational requirement for Class A:
  • Satisfy the specified time-temperature criteria through better operational control of the thermophilic stage of digestion. At a particular operating temperature, the digestion process should ensure a holding time determined from the time-temperature relationship. This equation yields detention times ranging from 24h at 55ºC to 12h at 57.5ºC.
  • Obtain approval from the EPA for a novel process as equivalent to Process to Further Reduce Pathogens (PFRP), based on a comprehensive demonstration of the process's capabilities to reduce pathogens to non-detectable levels.
    Efforts to demonstrate the 'equivalency' of advanced digestion schemes have proved to be an onerous and costly endeavour. Therefore, the key to fulfilling the requirements for Class A is to devise practical means for modifying the digestion process to achieve the specified time-temperature conditions for pathogen reduction.
    TPAD was identified as a suitable 'platform' for satisfying this objective. The TPAD process involves operation of the first stage of digestion at thermophilic temperatures (55ºC), followed by mesophilic operation (35ºC) in the second stage.
    When initially developed, the system was envisioned to operate in the semi-continuous mode, which does not satisfy the time-temperature criterion for Class A operational compliance. The thermophilic stage of digestion has to be operated in a batch, sequential batch, or similar mode to ensure that 'every particle' has been exposed to conditions stipulated under Class A. Operation in the sequential-batch mode (withdrawing a portion of the digester contents, refilling the digester, and holding for a period without further feeding) appeared to be a workable solution for eliminating short-circuiting concerns, but raised questions about process stability (Chao, 1999).

    Laboratory studies
    However, deviation from the intended continuous-flow mode of operation raised questions about the performance stability of the system. Laboratory studies at Iowa State University evaluated the performance and operational stability of a TPAD system modified to operate in the sequential-batch mode, retaining every 'particle' at 55ºC for 24h. The sequential-TPAD system was fed with a 40:60 mixture (dry weight basis) of primary sludge and waste-activated sludge at 5.5% total solids. With solids retention times (SRT) as short as 12 days in the system, the laboratory studies demonstrated that digestion process stability and performance are not compromised by sequential batch feeding:
  • The sequential-batch system did not show any effects of shock loading at any of the retention times studied. At 6-day
    thermophilic SRT, 33% of the reactor
    volume was discharged to the meso-
    philic stage every alternate day without upsetting the process.
  • Though the system performance was unaffected at shorter retention times, it took a longer time for the system to stabilise and attain steady-state. The system also showed some signs of instability after feed replenishment (every two weeks). It appears worthwhile to conduct some test runs at lower thermophilic retention times to determine the limiting SRTs for stable operation of the system.
  • The sequential-batch TPAD system achieved a volatile solids removal (VSR) of 49.5-53.4%, slightly higher than the
    43.1-43.7% observed on a 'conventional' single-stage mesophilic system operated at a longer SRT (14 days vs 16 days).
  • While the single-stage mesophilic system achieved only a 2-log reduction in the pathogen counts, sufficient to meet Class B standards (2M MPN/g TS), the sequential-batch TPAD system showed Class A performance compliance.
  • The enhanced VSR results in higher conversion of organic nitrogen, with ammonia in the effluent (1,810mg/L as N) from the TPAD system 20% higher than the single-stage mesophilic unit (1,570mg/L as N).
  • Methane recovery from the system ranged from 0.62-0.65L CH4/g of VS destroyed. The biogas contained trace amounts of hydrogen sulphide (150ppm from the thermophilic reactor and 25ppm from the mesophilic reactor).
  • At eight-day SRT in the thermophilic reactor, the intermediary volatile fatty acids (VFAs) were almost completely converted to methane in the first-stage reactor. However, at shorter retention times, significant amounts of VFAs were present in the thermophilic effluent, with the thermophilic reactor accounting for nearly 80% of the overall VSR and the mesophilic stage contributing the remaining 20%.
    While the TPAD system did not show any effects of shock loading and outperformed a 'conventional' mesophilic system operated at a longer retention time, performance stability is not the sole criterion for full-scale implementation. Critical applicability issues - such as digester heating, feed and discharge rates, and system configurations - have to be addressed.

    Process heating and feeding
    The energy requirement for thermophilic digestion is approximately twice that for conventional mesophilic digestion at the same feed rate. Heating loads, which are
    proportional to the feed volume, would increase proportionally if the digesters are fed over a shorter timespan. For a facility with limited digester volume, the sequential-batch feeding scheme would necessitate feed cycles of less than 24h duration, thereby increasing the energy input that would have been uniformly distributed over the 24h period if it were fed continuously.
    To moderate the heating loads for thermophilic digestion and make the overall digestion process energy-efficient, heat recovery would be an important feature in full-scale sequential-batch process designs aimed at Class A. With heat recovery, the net heating requirements for thermophilic operation could be reduced to levels comparable to mesophilic digestion. Thermophilic effluent at 55ºC could be used to partially pre-heat the raw feed sludge, which in some climates might be at temperatures as low as 10ºC.
    Post-thermophilic cooling is important for the operating economy, and to moderate the impacts on downstream materials-handling, in particular, high polymer demand for dewatering and more odorous biosolids.
    Heat recovery alone is not sufficient to meet the total heating loads, which are concentrated at the head of the digestion process to attain the process temperatures prior to beginning the digestor 'holding' period. Supplemental heating would be required to attain the thermophilic process temperatures in the limited time.

    Depending on the sludge production, the feed rate of raw sludge to digestion might vary. To maintain identical feed and discharge rates, it would be necessary for the drawdown phase to account for any anticipated changes in the quantity of sludge fed to the digesters. The intermediate pumping rate should track the pumping rate of raw sludge to digestion.

    System modifications
    Through conceptual development of the sequential-TPAD scheme, the following modifications have been identified for effective implementation of the process.
  • Heat exchange equipment - retrofitting existing digesters to operate in the sequential-batch TPAD mode would require the existing heat exchangers, originally designed for mesophilic operation, to be replaced or supplemented. By ramping up the operating temperature in the first-stage digester, the log mean temperature difference (LMTD), which defines the driving force for heat transfer, would be reduced below what the heat exchanger was originally designed for, reducing its output. New heat exchangers would have to be installed to accommodate sludge-to-sludge heat recovery and pre-heating requirements, whereas the existing units could be employed for recirculation heating (heat maintenance) in the digesters during the 'hold' phase.
  • Digester mixing - the appropriate mixing of digester contents plays an important role in the distribution of the heat and in ensuring uniform operating temperatures. Since draw-fill-hold would be the operating norm in sequential-batch operation, the digester-mixing systems need to be able to cope with varying liquid levels. While retrofitting the existing digesters, digester mixing could be limited to those times when liquid levels enable proper functioning of the mixers. Alternatively, pumped mixing, which is effective over a wide range of liquid levels, could be installed. In many cases, it may well be that instigating costly changes to replace the existing digester mixing system/equipment is not warranted.
  • Automation - the existing instrumentation and control capabilities would have to be improved to enable continuous monitoring of operating temperatures and other critical performance parameters for better operational process control. This would also ease the burden of increased complexity of sequential batching on plant-operators and provide documentation
    that the system is consistently operated
    in accordance with time-temperature requirements.
  • Piping and pumping modifications - in
    the draw-fill-hold operational scheme, the draw-phase from the thermophilic digesters could span over a shorter duration of time, requiring higher discharge rates which could consequently necessitate the replacement of the existing pumps. Piping modifications might also be required to accommodate changes in the flow pattern.
    As a result of the positive process and the economic evaluations, more than ten full-scale modifications of digestor plants to TPAD have been put into operation in the last few years, especially in the Midwestern United States, of which the largest, in Madison, Wisconsin, is under start-up to treat 30t DS/d.

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