Technically speaking

Black & Veatch's Frank Roalla looks at advanced digestion processes

Production of pathogen-free biosolids will be, in effect, a pre-requisite for land-based management schemes, as a result of more stringent regulations for the use of biosolids in agriculture in the EU and the Class A regulations in the US. Therefore, various technologies for upgrading digestor performance, particularly through process variations of conventional mesophilic anaerobic digestion, are being investigated.

The city plans to use a three-stage thermophilic digestion system, with the third stage operated in batch mode to satisfy the time-temperature requirements for Class A pathogen reduction. In early tests with Hyperion's large-capacity centrifuges, dewatering does not appear to be significantly impacted by thermophilic digestion. However, because of chemically enhanced primary treatment, the ratio of primary sludge-to-waste activated sludge (WAS) is still greater than 3:1. Early fecal coliform testing raised concern about consistently producing biosolids compliant with the 503 limit for Class A.

Two-phase digestion
Los Angeles County is no longer pursuing thermophilic digestion as plant-scale testing significantly impacted odours and polymer conditioning for dewatering. Most publicly-owned treatment works (POTWs) with an interest in advanced digestion are looking more seriously at coupled systems, with only one stage at thermophilic temperatures.

Mesophilic-mesophilic, two-phase digestion may increase volatile solids reduction (VSR) and digester gas production without the complications of temperature staging. But a general consensus has emerged that one of the two phases will need to be operated at thermopilic temperatures if Class A pathogen reduction is an objective.

Since July 2000, San Bernadino County, California, has been operating existing digesters in a three-stage mode that combines two-phase digestion and temperature phased anaerobic digestion (TPAD) - a mesophilic acid phase, followed by thermophilic and mesophilic methane-phase reactors in series. Elmhust, Illinois recently began operating a series of one-day batch acid reactors, followed by a mesophilic, methane-phase digester. The batch reactors were designed for filling over a 12h period and will initially be operated at thermophilic temperatures.

United Utilities recently completed studies of an acid-phase reactor to enhance pathogen reduction from mesophilic digestion, and is planning installation of a plug flow reactor (two-day solids retention time (SRT) at maximum loadings) at plant-scale. The main mechanism for pathogen kill was postulated to be enzymic hydrolysis, by which the enzymes are responsible for both cell lysis and hydrolysis. Mesophilic digesters operating in series achieved a 0.5 log increase in E.coli kill compared to a control digester operating at 16 days SRT. Pre-treatment in a three-day SRT acid-phase reactor resulted in a 1.8 log increase in E.coli reduction. Meso-meso, two-phase digestion may therefore be a good choice for those seeking to provide a greater margin of Class B compliance from existing digesters.

New facilities are under construction in Minnesota and Iowa as TPAD should be a good match for the feed solids characteristics at these plants. Midwestern US plants that previously converted their existing conventional digesters to operate in the TPAD mode are now trying to cool down the second stage (mesophilic) digester, in order to minimize impacts on dewatering and odour levels.

Sequential batch operation is one of the promising avenues for satisfying requirements for Class A biosolids and may be more easily incorporated in TPAD than a two-phase system. In August 2000, existing digesters at Birmingham, Alabama, were converted to enable operation in a sequencing batch, TPAD mode. However, the installation is designed for a batch holding time of 6 to 8h, considerably less than the 24h needed to satisfy the 503 time-temperature requirements at 55ºC. System operators will seek to satisfy the requirements by operating at higher temperatures approaching 60ºC.

Madison (Wisconsin) has been investigating various advanced digestion schemes and is now modifying existing digesters to enable operation in TPAD mode. Likewise, King County (Seattle) is beginning design for conversion of its 4-tank digester complex to TPAD, to accommodate higher future solids loadings within the existing digester infrastructure. King County recently completed an extended pilot plant investigation of TPAD for its West Point treatment plant, where there is neighbourhood pressure to reduce the number of digesters.

Exploratory work
With the uncertainties of satisfying Class A requirements with continuous-flow two-phase digestion and TPAD, pre-pasteurisation is thought of as an add-on to conventional anaerobic digestion, to provide a more certain route to achieving pathogen reduction. Accordingly, it has been drawing greater interest for meeting newly enacted enhanced treatment requirements in the UK and EU. Monsal and BHR Group have collaborated on some pioneering work in exploring the relationships between sludge rheology, mixing and heat transfer as applied to pasteurisation of highly thickened biosolids.

Alexandria, Virginia has opted to add pre-pasteurisation facilities for achieving Class A pathogen reduction as a high-priority. There are a number of proprietary packages for pre-pasteurisation, based on a batch-operating concept. Recently, the Eastern Municipal Water District (California) and Ashbrook have teamed up to offer a pre-pasteurisation package known as Eco-Therm, based on a continuous, plug flow concept.

Cambi is a proprietary, thermal hydrolysis pre-treatment process, to be used in conjunction with conventional anaerobic digestion. Pasteurisation is an incidental benefit of using the Cambi process for downstream dewatering, with the main objective being to reduce the required volume of new digesters.

The wastewater industry is waiting for a consensus on which of the advanced digestion process options will emerge as the optimum alternative. Experimentation shows that anaerobic digestion appears remarkably resilient, contrary to its reputation of 25 - 30 years ago. At least at lab and pilot-scale, stable operation has been reported at low SRTs.


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