Treating the wash water of a public transport vehicle
Public transport operators pay increasing attention to keeping their vehicles clean. The water from washing must be treated before it is discharged into the public sewer. Our company installed a process in which the wash water is also recycled and reused: only treated water of a quality that complies with the regulations is discharged.
The washing process
The buses are washed by machine. As in the familiar car washes, a sequence of different programmes runs here as well:
- pre-wash, soaking
- rinsing
- chemical washing
- rinsing
- drying
A chassis wash can run in parallel with this and may optionally be part of the wash programme. The run-off water – especially when the chassis is washed too – may contain oil, oily emulsion, detergent water and suspended solids (sand, mud), all of which must be removed before discharge to the public sewer.
The solution implemented by our company saves water, because part of the treated water can be reused for washing. In this case the final rinse is always done with mains water, and only the treated water that is not reused reaches the sewer outlet.
Pictures from the project
The vehicles are washed in a so-called wash corridor, through which they pass slowly. Depending on the programme, the body and the chassis are washed. This uses roughly 200–400 l of water per vehicle, 80–85% of which is recycled water. Part of the wash water is carried away on the body and part evaporates; the make-up is mains water. Washing one vehicle takes about 2–3 minutes, so up to 20 vehicles per hour can be washed, depending on the programme.
Pollutant load considered during design
| Parameter | Value |
| pH | 6–8 |
| COD (chemical oxygen demand) | 1000–2000 mg/dm³ |
| Solvent extractable matter (oils, fats) | 100–200 mg/dm³ |
| Suspended solids | 200–500 mg/dm³ |
Table 1: Expected quality of the wastewater to be treated
Required quality of the treated wastewater
After leaving the treatment technology, the treated water is mixed with rainwater and flows into the lifting basin built in the yard, then into the storm reservoir, and from there through a control point into the public sewer. The treated wastewater is recycled: only the surplus is discharged, and its quality meets the limit values for wastewater dischargeable into the public sewer laid down in Annex 2 of Hungarian Ministerial Decree 28/2004 (XII. 25.) KvVM.
| Parameter | Limit value |
| pH | 6,5–10 |
| COD | 1000 mg/dm³ |
| BOD₅ | 500 mg/dm³ |
| Solvent extractable matter | 50 mg/dm³ |
| total nitrogen | 150 mg/dm³ |
| mineral oils | 10 mg/dm³ |
| matter settling in 10 minutes | 150 mg/dm³ |
Table 2: Limit values of the relevant pollutants
Description of the wastewater treatment technology
The vehicles are usually washed on the evening and night shifts, and the washing operations take at most 8 working hours. The hydraulic capacity of the water treatment system was determined to match these requirements.
| Parameter | Value |
| Hourly wastewater generation | 12 m³/h |
| Daily wastewater volume (at 300 l per vehicle and 200 vehicles per day) | 60 m³/day |
| Daily mains water consumption | 9 m³/day |
| Daily water recycled | 51 m³/day |
| Calculated water loss | 15% |
Table 3: Wastewater load from the exterior washing of the vehicles
Wastewater load from other sources: interior washing accounts for 0.5–0.8 m³/day, while the sump of the maintenance hall yields 2–3 m³ per emptying, on a quarterly basis.
To maintain continuous and safe operation, we built a continuously operating, automatic wastewater treatment unit with a capacity of Q = 12 m³/h.
Process flow diagram of the designed solution
Operation of the plant
The wash water is collected in an underground tank of V = 25 m³, designed to retain both settled sediment and floating debris, which have to be pumped out from time to time. The contaminants in the water are removed by chemical dosing: the polyaluminium chloride type inorganic coagulant neutralises the negative charge (zeta potential) that forms at the oil-water colloidal interface, and the contaminants are adsorbed on the surface of the resulting flocs, which have a large specific surface area.
A flocculant is dosed to increase the size of the sludge particles, producing larger flocs. Where necessary, sodium hydroxide is dosed in the second reactor stage under pH control, so that the pH stays within the 6.5–9 limit even if the wastewater turns acidic.
The flocs formed during the chemical treatment are separated on filter units with a multimedia bed. The treated water passes from the flocculator tank directly to the sand filter, from where the clean water reaches the collecting-storage tank. The high-pressure pump serving the washing machine draws from this tank – this is how the water is reused.
When the level in the clean water tank reaches the overflow, the treated water is diverted into the discharge funnel connected to the yard basin; the occasional complete water change of the system also reaches the public sewer along this route. If there is not enough treated water in the storage tank for washing, make-up water is supplied from the works water main under level-switch control.
The backwash water of the sand filters is led to a sludge thickening tank, where the water phase is removed by automatic decanting; the sludge is dewatered on a plastic chamber filter press.