Bridge over OILY WATER

Making jet engines might not seem the dirtiest of businesses, but it leaves Volvo Aero Corporation's plant at Trollhattan, Sweden, with 20000m3 of dirty water to clean up every year.

Volvo makes parts for jets in collaboration with GE, Rolls Royce and BMW. The waste waters mainly originate from three sources: oil emulsions from machine tools, rinse waters and process liquids from water-based cleaning, and liquid penetrants used for crack testing.

Volvo's first attempt to clean up its act involved installing an ultrafiltration (UF) plant to concentrate the oily residues. This was only partially successful, as the chemical oxygen demand levels in the membrane filtrate remained between 2-6g/l, and the residue remained toxic.

The solution was to install a reverse osmosis plant to pre-concentrate the permeate from the UF plant, followed by a forced recirculation evaporator from Stork Engineering. This combination minimises the volume of material for disposal and cleans the water effluent enough for discharge to drain.

Contaminated waste waters vary in nature, concentration and volume and arise from all parts of the site, but the evaporator feed generally contains less than 1 per cent dry solids. The purpose is to deliver clean, non-toxic water of COD below 200mg/l, along with a polluted effluent stream, having above 50 per cent dry solids, for off-site disposal.

The Stork evaporator features forced recirculation in a tubular heat exchanger and is designed as a non-fouling operation so that no cleaning is required. It has a capacity to treat 30000m3/y of waste waters, and can function in both batch mode and continuously.

The feed stream flows through pipes in a heat exchanger where its temperature is raised by steam. The system is pressurised to prevent boiling. At the entrance to an expansion chamber, the pressure is suddenly released, which flashes off much of the water as steam and concentrates pollutants in the remaining liquid.

Much of the energy input to the heat exchanger is recovered from the flashed steam by mechanical vapour recompression. Two fans are installed, which may be used separately or in series depending on required capacity. The evaporation is done in one effect and power consumption in this phase is about 25kWh/m3 of evaporated water.

Internal recirculation of concentrate is adjusted by a pump, to stabilise a flow-rate between 300-450m3/h. Design features of the evaporator include a separator whose efficiency on entrainments is better than 99.99 per cent. Nevertheless, since the feed contains volatile hydrocarbons, activated carbon must be used for condensate treatment. Other conditioning possibilities include reverse osmosis, ozone, hydrogen peroxide or aerobic digestion.

At Volvo, the final outflow has less than 200mg/l litre COD, of which between 100 and 150mg/l is BOD, and a pH around 8-9.

FORGET ABOUT FOULING

The high temperatures and pressures prevailing in the heat exchanger raise a risk of overheating the waste stream, which could cause fouling. Detailed design ensures that temperature of the hottest part of the internal tube wall stays below the critical point where polymerisation, dissociation or breakdown might begin to cause problems.

Steps are also taken to eliminate a risk of foam formation in the expansion chamber, using a deaeration technique.

Operation at Volvo Aero Corporation is in principle continuous - and is able to remain so thanks to the evaporator's non-fouling design - although during weekends or when contents of the 1000m3 storage tank are at a low level the system can be set in idling mode. This means that the recirculation rate is kept low, and compression is suspended.

Discounting the special environmental regulations in Sweden, it is possible to envisage a waste water feed stream to the evaporator comprising roughly 95 per cent water and 5 per cent oil. At the outflow these proportions could be reversed. The oil can be concentrated so much that it could be used to fuel an oil-fired boiler.

This evaporation technique could also be used in electroplating plants, where waste waters are conventionally treated by precipitating hydroxides of heavy metals. In place of further processing by flocculating or filtering to reduce the bulk for disposal, a forced recirculation evaporator can yield a highly concentrated sludge in one operation, together with a purified effluent acceptable for discharge directly to the drain.

Tony Robinson is sales engineering manager at Stork Engineering

STORK ENGINEERING ENTER 270