Wastewater treatment of a surface treatment plant
Year of completion: 2018
Several plants in the Industrial Park carry out similar activities – manufacturing automotive components and household appliances – so the wastewater they generate is similar in character. We therefore built a shared wastewater pre-treatment technology to handle the treatment. A key element of the design was the selective drainage and collection of effluents with different contamination levels (rinses, concentrates, ion exchange regenerates, etc.).
The treatment technology is based on precipitation following pH adjustment. After the sludge and water phases are separated, the water goes to ion exchangers and the sludge is passed to a mechanical dewatering unit. The treated water reaches the existing biological stage – this was not part of our scope.
Detailed description of the solution
The plants operating in the Industrial Park manufacture various components as automotive suppliers. Their subsequent processing (welding, enamelling, powder coating, etc.) requires surface preparation – this is what delivers the corrosion resistance, weldability and surface adhesion of the finished products, and ultimately their longer service life.
The surface treatment operations that generate wastewater are the following (their order and number may vary):
- degreasing
- rinsing
- sulphuric acid or alkaline pickling
- rinsing
- coating
- rinsing
The volume of wastewater to be treated is Q = 135 m³/day. The guiding principle of the pre-treatment design was to collect waters of different contamination levels separately and then feed them to the treatment line in controlled quantities. In this way the continuously generated, lightly contaminated rinse waters and ion exchanger wash waters, as well as the concentrated standing baths drained periodically (weekly or monthly), reached the treatment line selectively.
| Step | Operation | Chemical / medium | Concentration (g/l) | Treatment time (min) | Bath volume (m³) | Temperature (°C) |
| 1. | degreasing | Bonderite C-AK 1372 | 40–50 | 15–20 | 0,7 | 60–70 |
| 2. | rinsing | tap water | max. pH 10 | 1–3 | 0,7 | 40–70 |
| 3. | sulphuric acid pickling | sulphuric acid | 130–180 | 15–20 | 0,7 | 50–70 |
| 4. | acidic rinse | deionised water | min. pH 4 | 1–3 | 0,7 | 18–22 |
| 5. | sulphuric acid pickling | sulphuric acid | 80–120 | 3–5 | 0,7 | 18–22 |
| 6. | rinsing | deionised water | min. pH 4 | 3–10 | 0,7 | 18–22 |
| 7. | rinsing | Na₃PO₄ / NaNO₂ | 3–4 / 1–2 | 2–5 | 0,7 | 50–70 |
| 8. | rinsing | deionised water | pH 3–10 | 1–3 | 0,7 | 18–22 |
| 9. | degreasing | Bonderite C-AK 1372 | 15–25 | 5–15 | 0,7 | 40–70 |
| 10. | rinsing | deionised water | max. pH 10 | 1–3 | 0,7 | 50–70 |
Table 1: Characteristics of a surface treatment bath line
Pictures from the project
The essence of the chemical treatment
The dissolved metal content (Zn, Mn, Fe, Al) and the fluoride, sulphate and phosphate content are precipitated by lime milk treatment: the dissolved metals precipitate as hydroxides, the phosphate as calcium phosphate and aluminium phosphate, the sulphate as calcium sulphate and the fluoride as calcium fluoride, all passing into the sludge phase together with the other contaminants. The processes are accelerated with an aluminium-based water treatment agent (Aquapac). Beyond precipitating the metals as hydroxides, the aluminium hydroxide flocs that form help to further reduce the dissolved contaminants and the grease and oil content.
The main contaminant precipitation reactions taking place during the treatment:
Al3+ + PO43− → AlPO4
3 Ca2+ + 2 PO43− → Ca3(PO4)2
Ca2+ + 2 F− → CaF2
Ca2+ + SO42− → CaSO4
By neutralising the wastewater, the heavy metal and metal ions can be precipitated according to the following reaction equations:
Fe3+ + 3 OH− → Fe(OH)3
Al3+ + 3 OH− → Al(OH)3
Mn2+ + 2 OH− → Mn(OH)2
Zn2+ + 2 OH− → Zn(OH)2
| Parameter | Value |
| pH | 4–12 |
| COD | 100–3000 mg/dm³ |
| Solvent extractable matter (oils, fats) | 100–500 mg/dm³ |
| Total manganese | 1–2 mg/dm³ |
| Total fluoride | 1–2 mg/dm³ |
| Total sulphate | 10–500 mg/dm³ |
Table 2: Expected composition of the wastewater to be treated
The main process steps
- selective collection and equalisation of the individual effluents, as required;
- selective storage of the concentrates (acidic, alkaline);
- feeding to the treatment line at flow rates matching the calculated loads;
- pH adjustment;
- chemical treatment with reagent dosing;
- separating the resulting sludge phase from the water phase by sedimentation;
- dewatering of the sludge with a chamber filter press;
- post-treatment of the water phase on ion exchange columns;
- discharge through a final control shaft.
Process flow diagram
| Component | Discharge limit value | Type |
| pH | 6,5–9 | area limit value |
| Dichromate oxygen demand (COD_Cr) | 75 mgO/l | area limit value |
| Biological oxygen demand (BOD₅) | 25 mgO/l | area limit value |
| Ammonia-ammonium nitrogen | 10 mg/l | area limit value |
| Total inorganic nitrogen | 40 mg/l | area limit value |
| Total nitrogen | 50 mg/l | area limit value |
| Total suspended solids | 50 mg/l | area limit value |
| Total phosphorus | 5 mg/l | area limit value |
| Solvent extractable matter (oils, fats) | 5 mg/l | area limit value |
| Total nickel | 150 µg/l | individual limit value |
| Total cadmium | 1,2 µg/l | individual limit value |
| Total zinc | 400 µg/l | individual limit value |
| Total chromium | 150 µg/l | individual limit value |
| Chromium (VI) | 100 µg/l | individual limit value |
Table 3: Limit values prescribed in the integrated environmental permit
Our scope of supply
The project was delivered on a so-called turn-key basis. Our scope of supply covered the following:
- on-site survey of the wastewater discharges of the production processes;
- laboratory treatability tests;
- process design;
- preparing the documentation required for permitting;
- procuring and/or manufacturing the machines and equipment needed for the technology;
- installation of the technology;
- commissioning and operator training;
- professional supervision of the trial operation and preparing the trial operation final report;
- preparing the documentation for the operating permit.
The wastewater treatment technology runs automatically: it is controlled by a central PLC with the help of level switches, sensors, motorised valves and pH probes. The project was completed in 2018.