Treating process reject water using the Amtreat Process

Bungay, S.1 and Collins, S.2 1Helix ECL, UK, 2 ACWA Services UK,


The Amtreat process is a high-rate activated sludge plant for treating high-strength ammonia
liquors with typical ammonia removal rates in excess of 97%. The original Amtreat process was
developed in 1992, with the first full-scale plant installed at Cliff Quay STW in 1998 for Anglian
Water. The plant was designed for complete ammonia removal. The second plant was installed
at Ashford STW in 2008 for Southern Water. This plant was configured for complete ammonia
removal and partial denitrification and alkalinity recovery.

ACWA are currently designing three new Amtreat plants for Anglian Water at three advanced
digestion sites. The flexibility of the process has allowed two sites to re-use existing assets;
including upgrading of the original plant at Cliff Quay. These third generation plants are
designed to maximise nitrification, and are designed with glycerol and sodium hydroxide dosing
to optimise denitrification and alkalinity recovery.

This paper discusses the development of the Amtreat process, and the design and operation of
the Amtreat Plants at Cliff Quay and Ashford.

Ammonia Removal, Amtreat, Liquor Treatment, Nitrification, Denitrification, Alkalinity Recovery.


Conventional Mesophilic Anaerobic Digestion (MAD) produces a digestate that typically contains
aqueous ammonia concentrations of 500 – 1,000 mg/l. If the digestate is dewatered to produce
a caked product, the aqueous ammonia will be present in the dewatering process reject water
or liquor. This reject liquor may represent 15 to 25% of the load.

As the number of municipal Advanced Anaerobic Digestion (AAD) and commercial Biowaste
Anaerobic Digestion (BAD) plants have increased, there has been an associated increase in the
ammonia concentrations contained in the process dewatering liquors. At AAD plants, the
anticipated concentration of ammonia is in the range 1,500 – 3,500 mg/l, whereas at BAD plants
ammonia concentrations in excess of 5,000 mg/l have been encountered.

At municipal Sewage Treatment Works (STW), the traditional treatment strategy for these liquors has been to pass them back to the existing treatment works. However, with the increasing implementation of tighter and total nitrogen limits, and the increasing ammonia concentrations, this practice may not be feasible because such plants often have insufficient nitrifying capacity. Commercial BAD plants tend to be stand-alone plants, so there is not a STW to return the liquors to. Therefore, in both applications there has been a move to treat reject liquors at a separate, dedicated plant

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