Two-stage wastewater treatment technology

Two-stage wastewater treatment technology

For treating waters with high salt content and oily emulsions

Wastewater from vehicle washing typically contains oil and oil emulsion contaminants as well as suspended solids (sand, mud), which also appear as dissolved salts in the water. We used the conventional coagulation-flocculation method for the oily-emulsion contamination and removed the dissolved salts by membrane filtration in a second treatment step.

Origin and characteristics of the wastewater

At the site, railway wagons are cleaned inside and out. The interior cleaning essentially means collecting refuse and generates no wastewater. The exterior washing is basically done without chemicals; detergents are used only in exceptional cases. The high-pressure washer uses about 400–800 litres of water per wagon. The area around the purpose-built washing tracks is paved, the wash water is drained in a closed channel, and coarse contaminants are removed on a filter with a 1.5 mm slot size. Rainwater is drained through the same system.

Table 1: The main pollutants and their characteristic values

The treated wastewater is discharged into the public sewer and its quality had to meet the limit values for wastewater dischargeable into the public sewer laid down in Annex 4 of Hungarian Ministerial Decree 28/2004 (XII. 25.) KvVM, as well as the requirements set out in the receiving party's declaration.

Table 2: Limit values of the relevant pollutants

The wastewater treatment technology

Laboratory checks and treatability trials showed that the chemical treatment used to remove the oily contaminants does not on its own keep the total nitrogen – and, accordingly, the salt content – within the limit values. We therefore designed the following two-stage treatment:

1. Stage I: removal of the suspended solids and the oily contaminants by chemical treatment;
2. Stage II: removal of the dissolved salts by membrane filtration.

To maintain continuous and safe operation, a continuously operating, automatic wastewater treatment unit with a capacity of Q = 2 m³/h was built. The wastewater to be treated – the wash water and the rainwater – flows into an underground collecting pit. Conductivity and turbidity probes were built into the discharge line of the feed pump installed there: if the measured values are below the preset thresholds, the pump sends the water directly towards the public sewer by automatically operating the appropriate valves.

At higher measured values the wastewater is directed to the treatment line. Here a coagulation-flocculation reagent is first dosed in a stirred reactor: as a result, the oily and other contaminants and the suspended solids bind to the surface of the precipitate that forms and pass into the sludge phase. Acid/alkali dosing controlled by a pH probe is also possible in the reactors where necessary. The sludge and water phases are separated by sedimentation and the sludge is dewatered on a plastic chamber filter press.

There are two options for discharging the treated water:

• if the salt content measured by conductivity is below the preset value (corresponding to 2500 mg/l), the water flows through the clean water tank into the public sewer;
• if the conductivity is higher, the water is routed to treatment stage II, the membrane filter.

Process flow diagram

Using the membrane filter (RO – reverse osmosis unit), the concentration of dissolved salts can be reduced on the permeate side, because the salts are concentrated in the concentrate leaving the membrane. This concentrate must be handed over to a duly licensed company for further treatment and disposal; the permeate can be discharged into the public sewer.

Related fields of expertise

• Industrial wastewater treatment
• Water treatment and conditioning
• Plastic chamber filter press
• Sludge dewatering
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