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.
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
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
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
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
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.
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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