Main biological treatment stage

The sensitivity to pH is another aspect not always considered. Homogeneous Fenton processes need to operate in a narrow pH range (2.5. 0) and this is a main process issue, as it requires the addition of large amounts of salts to the wastewater for pH control. The use of solid Fenton-type catalysts allows, in principle, operation at natural pH, without the need to adjust the pH, with better process economics. In addition, solid Fenton-type catalysts avoid the need for precipitation of iron and remove the iron sludges after treatment. This is a relevant process issue, especially when the operation is a pre-treatment before further stages such as biological treatments. 

The MBR takes the place of the settlement stage after secondary treatment, which reduces the organic content of the waste liquor by biological action. The separation is achieved by ultrafiltration membranes, in hoUow fibre or sheet formats, submerged in the main secondary treatment vessel, and often with air bubble scouring of the outer surfaces to extend operating times. 

Techniques for in situ sediment treatment are still in an early stage of development, and the very few methods currently commercially available are usually restricted to situations where the low water levels allow for water flow diversion. Potential in situ treatment methods include biological, chemical, and stabilization processes. The main advantage relies upon avoiding sediment removal and, therefore, the related risk of spreading the contamination. However, technical limitations to the application of in situ treatments still exist, mainly because the effective delivery of such process reagents as substrates, nutrients, and chemicals, as well as mixing with the contaminated material, may be problematic. 

In Japan a certification system has been started by the Biodegradable Plastics Society (BPS), an industry association on biodegradable plastics, which in many aspects is different from the European and American systems. The main focus of the system is biodegradability and environmental safety. Compatibility with a typical biological waste treatment system or disintegration within a specific time period is not an issue at this stage. 

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