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).
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:
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.
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:
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).
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:
thermophilic SRT, 33% of the reactor
volume was discharged to the meso-
philic stage every alternate day without upsetting the process.
43.1-43.7% observed on a 'conventional' single-stage mesophilic system operated at a longer SRT (14 days vs 16 days).
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.
Through conceptual development of the sequential-TPAD scheme, the following modifications have been identified for effective implementation of the process.
that the system is consistently operated
in accordance with time-temperature requirements.
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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