Incrustations in municipal waste water treatment plants
In municipal waste water treatment plants with sludge digestion, deposits are more often observed in pipes, pumps and sludge dewatering units. This effect is particularly common in centrifuges and their centrate discharge pipes, sludge feed lines to dewatering equipment and in filtrate discharge lines of chamber filter and belt presses. Constricted pipes can cause a backflow of filtrate or centrate. Pumps and centrifuges are heavily exposed to increased wear. Often, the deposits are resilient enough that ordinary pipe cleaning systems cannot be used successfully. In such a case, the only options left are disassembly and use of expensive mechanical cutting devices or manual mining techniques and replacement of the affected pipes.
The cause of incrustations is the supersaturation of the aqueous solutions with the ions involved in the formation of crystals. In municipal waste water treatment plants, deposits of magnesium ammonium phosphate (MAP) are particularly frequent. While magnesium is present in virtually all waste waters, large amounts of ammonium and phosphate are released in the process of sludge digestion. The precipitation of MAP noticeably starts above pH 7.5. The pH of the sludge water, which is between 6.5 and 7.5 after digestion, shifts into an alkaline range thus causing incrustations to occur. The onset of this negative effect is generated by spontaneous outgassing of carbon dioxide formed during the digestion. The carbon dioxide is instrumental in the formation of a buffer system in the sludge water. Such decompression processes are preferably carried out in centrifuges and pump housings under the influence of strong turbulence.
The following chemical compounds are the most common deposits in municipal waste water treatment plants:
- Magnesium ammonium phosphate (struvite) MgNH4PO4 x 6 H2O
- Calcium sulphate (gypsum) CaSO4 x 2 H2O
- Calcium carbonate (limestone) CaCO3
- Calcium phosphate (hydroxyapatite) Ca5(PO4)3OH
With the aid of scanning electron microscopy coupled energy dispersive X-ray microanalysis (SEM-EDX), elemental analysis is done in a single microscopic examination. The crystalline structure, possible layer build-up of the deposits and the chemical composition can be determined with sufficient accuracy from the examination.
A targeted metering of special anti-incrusting agents effectively prevents the formation of different incrustations. Polymeric sodium acrylates are used to disturb the crystal growth of precipitating salts so that solidification is prevented. The insoluble compounds are transported away with the water without any further problems. Suitable dosing points for these anti-incrusting agents are in the regions where the crystals occur. These are often the centrate or filtrate outlets of the dewatering units. However, if deposits occur in sludge pipes, a dosing into the discharge pipes of the anaerobic digester may be necessary. The dosing amounts are in the range of 10 – 20 ppm. These are to be determined using the actual application. For dosing, simple diaphragm pumps are used. Often, technological changes in the waste water and sludge treatment conditions can suppress certain occurrences of incrustation in municipal waste water treatment plants. We would be pleased to advise you.
Other laboratory services:
- Waste declaration analysis
- Determination of dioxins in sewage sludge
- Analysis of sewage and biogases
- Corrosion tests on components of drinking water and waste water pipes
- General waste water and sludge examinations
- Material studies using light microscopy, SEM and TEM
- Asbestos investigations in structures and in the air
All tests are conducted in accredited laboratories.