Separate but equal

The regulation was designed to control the disposal to landfill of hazardous substances and included a total ban on the disposal of liquids. Liquids present particular problems for landfill sites in the short term, exceptional load on the sites’ leachate treatment systems and long term they pose fugitive liability problems due to over saturation of the site.

Until now, liquids and sludge of all kinds were allowed to be disposed of to landfill. On 21 June 2002 that came to an end, overnight.

These were typically process liquids from a wide range of industries, including special and non-special wastes, spent process product wash down liquors, filtrates, scrubber liquors, wash outs, soluble oils, paint spray booth water, interceptor wastes, etc. In fact everything that was not considered hazardous or flammable.

Statistics indicate that liquids disposed of to landfill exceeded 1.03m tonnes of liquid 0.56mte as non-special and 0.47mte meeting the special waste status classification in the year 2000-2001.

So what options are left to producers that still need to dispose of liquid wastes? Fundamentally, all actions need to fit in with the basic hierarchy of good waste management practice: minimise, to reduce volumes as much as possible; re-use, within the process as far as possible; treatment to reduce its level of toxicity; incineration or thermal destruction with consideration for energy recovery and waste that cannot be treated in any other way. Landfill is at the very bottom of this hierarchy.

Treatment can include various forms at both off-site merchant facilities or onsite bespoke solutions. Physio-chemical or biological, for example, Also, neutralisation of an acidic waste stream or introduction of the effluent to biological nutrients to encourage aerobic bacteria to break down organic matter in the waste stream as the latter option.

Waste consolidation

A further route for waste streams at some merchant facilities promotes the term ‘consolidation’ as a form of treatment. This involves the mixing of liquid wastes with other absorbent material or waste to produce a solid and in the majority of cases this does little to reduce the toxicity of the waste stream.

Currently the description of a ‘solid’ is very loose. New regulations, now at the consultation stage, will clarify what is needed and define waste in terms of its Total Organic Carbon (TOC) as is currently the case in France and Germany. The consequence is consolidation, accepted as being a way around the regulations rather than within their spirit, will soon be outlawed or impractical.

The Westfalia system for dewatering liquids using decanters to create a 28-35 per cent total solids ‘cake’ is both practical and efficient. By including an element of flocculation it is possible to obtain a water stream that is sufficiently pure to be discharged to the sewers or returned into the factory’s process.

As always there are alternatives. Belt filter presses, for example, perform a similar function but are less efficient, are labour intensive and have a potential health and safety issue as much of the residual material often has to be scraped from the press manually.

Gravity separation tanks are another solution but settlement is slow and there is a risk of contamination between the separated phases.

One problem aimed at decanters was always that they lacked flexibility: they could not react to varying feed densities. However, the new ‘Crocodile’ decanters from Westfalia have a ‘2-motor drive’ permitting torque-dependent control of the differential speed that senses the density of the feed and adjusts the power settings accordingly to maintain a consistent separation quality.

IPPC

Under Integrated Pollution Prevention and Control (IPPC), to which prescribed industrial processes are required to comply, all emissions to air, water and land must be declared and quantified. ‘Best Available Techniques’ (BAT) must be sought and these are to be determined by the use of low waste technology, the use of less hazardous substances, adopting technological advances and changes in scientific knowledge and understanding will satisfy BAT criteria.

Dewatering liquid wastes, using modern decanters helps companies comply with these requirements, fits with the hierarchy of works management by reducing the volume for disposal, and satisfies the demands of the June 2002 regulations. In turn this pays for itself through savings in transport and reduced disposal fees.