Simplifying sludge treatment

By Piers Clark
WS Atkins Water

June 29, 2001--Water companies face increasing public and legislative pressure to attain high standards of sludge treatment. This job is further confounded by confused messages from European Union directives informing of the need to both recycle and reduce incinerator use and subsequent emissions, whilst paradoxically, producing sludge so clean that most natural soils would fail to comply.

Water companies are adopting a comprehensive improvement programme to manage these often-conflicting demands. These include cleaning anaerobic digesters, refurbishing them and revamping operational protocols to incorporate Critical Control Points and audit procedures. However, such comprehensive approaches on some sites may still not be enough to meet the desired exit sludge quality criteria.

WS Atkins is developing a treatment method that solves the sludge problem by using a group of environmentally safe biocides. These compounds, referred to as Borate-carrier biocides (BCBs), have considerable potential to reduce pathogens in sewage sludge. WS Atkins is leading a programme to develop this solution, working with UK companies Ectopharma Ltd and Micro Science technologies Ltd. Four UK water companies are funding this project: Yorkshire Water, Anglian Water, Southern Water and Severn Trent Water.

How do the BCBs work? The naturally occurring mineral borate is toxic to a variety of microbial and insect pests. The BCBs consist of borate linked to an organic carrier molecule, which enhances greatly the intrinsic toxicity of borate.

BCBs were originally developed in a research programme aimed at replacing organophosphates, currently used in sheepdip and in medicated shampoo preparations with environmentally friendly alternatives. This was their limited use until now.

BCBs are highly toxic towards target organisms in sewage sludge, highly biodegradable and low in toxicity towards humans. Expected benefits are:

* rapid and highly effective sterilisation;

* safe to handle and stable in storage;

* completely biodegradable, little environmental impact;

* inexpensive to produce and apply.

Initial results show that BCB compounds have considerable potential to ensure that sewage sludge complies with proposed strict European-wide microbiological standards.

Conventional sludge treatment processes remove about 99% of sludge pathogens. A further 90% to 99% of remaining pathogens can be destroyed by adding BCB.

Currently, WS Atkins is undertaking a scale-up programme of development work that involves detailed testing of BCB compounds and working with established polyelectrolyte suppliers to develop a practical and effective dosing regime.

Full results are expected by the end of summer 2001. The microbial standards legislation came into effect in January 2001 so sludge treatment innovations are needed immediately. Research and development continues, but WS Atkins and funding client organisations are striving to implement this technology as soon as possible. This breakthrough is expected to make the handling and disposal of sewage sludge by water companies more easy and efficient.