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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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