Treatment of sewage not susceptible to biological decomposition

The Fenton reagent (H2O2 : Fe (II)) is used for the treatment of industrial sewage unsusceptible to biological decomposition. Fe (III) ions can also be used. The Fenton reaction leads to the creation of hydroxyl radicals with a very high redox potential (2.8 V), which react non-selectively with many organic compounds. For the process to be effective, a pH of 3-5 must be maintained. The reaction time is limited by the speed of hydroxyl radicals’ creation, which in turn depends, among others, on the catalyst concentration (i. e. Fe (II)) and the hydrogen peroxide concentration (specifically, the Fe (II) : H2O2 ratio). The ratio is usually 1:5 ÷ 10, assuming Fe2+ concentration of 3-50 mg/dm3. Reagents can be added all at once or sequentially. The Fe2+ dose and the Fe (II) : H2O2 ratio must be previously defined experimentally. Reaction time usually varies between 30 minutes and 24 hours. The reaction can occur in flow-through or fully mixing reactors, powered permanently or temporarily.

 

The usage of Fenton process is justified if COD concentration is bigger than 500 mg O2/dm3 and the BOD5 : COD ratio is low (<0.2). Fenton reaction can be used for both wastewater treatment (full oxidation of organic compounds to CO2 and H2O) and pre-treatment. The aim of sewage pre-treatment is the oxidation of compounds unsusceptible to biodegradation to simpler, easier to degrade combinations. The suggested temperature is 20-30 ºC (temperature increase above 40-50 ºC lowers the effectiveness of the oxidation process). The reaction is also inhibited by pH above 10, due to H2O2 decomposition. The Fenton reagent also enables the effective removal of carcinogenic and toxic organic micropollutants, as well as odour and colour from sewage. It can also be used for sludge conditioning and ground remediation.

 

Phosphorus removal

If the main issue with a wastewater treatment plant is the low effectiveness of biological phosphorus removal, an additional physical-chemical methods can be implemented. In practice, this means using the processes of coagulation and orthophosphate precipitation.

 

The method consists of four main stages: precipitation, coagulation, flocculation and separation of solid particles.

 

The first stage is mixing the precipitation reagent with wastewater. This process leads to the transformation of orthophosphates diluted in sewage into metalloid phosphates.

 

The second stage involves coagulation, which is the process of neutralizing the surface charge of sewage particles and creating hydroxides. Metal ions cause the creation of bigger hydroxide complexes (flocs), which bind the precipitated metalloid phosphates and other suspended materials, thus lowering the COD and BOD indices.

 

The third stage is flocculation, which enables flocs to merge into bigger structures called agglomerates.

 

The last stage is solid particles removal, during which chemical sludge is eliminated from the system.

 

The chemical phosphate precipitation technology is broadly used in many wastewater treatment plants and serves as a good supplement of the biological processes. It increases the effectiveness of phosphorus removal with relatively low investment costs and simple implementation. The usage of chemical support increases the amount of sludge created in the plant and the amount of metal ions in the sludge.

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