They are released to the supernatant and increase the COD value. According to Mustranta and Viikari (1993), as a result of polymer degradation, biodegradable compounds of relatively low molecular weight are formed. At larger doses, it effectively oxidizes the sludge polymers ( Mustranta and Viikari, 1993). It only oxidizes organic compounds present in the supernatant or makes them more available for rapid degradation. At low doses, Fenton's reagent does not oxidate and destroy the polymers present in the sludge. In the studies by Barbusiński and Filipek (2000), relatively low doses of Fenton's reagent were used. Studies made by Barbusiński and Filipek (2000), showed that low doses of Fenton's reagent decrease COD of supernatant. Fenton's reagent can change COD of reject water. In the scientific literature, it is known that chemical conditioning/disintegration of sewage sludge affects not only the properties of dry matter but also the composition of reject water. These polymers constitute the relevant cohesiveness of sludge floc and affect the way in which they bond with water ( Barbusiński, 2010). The most important processes are probably oxidation and destruction of extracellular polymers, which are components of activated sludge flocs.
The mechanism of the improvement of specific resistance to filtration has not been sufficiently explained yet. It should be emphasized that when we disintegrate the sludge under acidic conditions, the additional leaching of such micropollutants as heavy metals can occur ( Janosz-Rajczyk and Wisniowska, 2005). pH affects not only process effectiveness but also has an effect on the fates of other pollutants. A difference in the specific resistance to filtration rate of 90% was stated in favor of pH 2.6. (2001) showed that changes in the rate of specific resistance to filtration depend on pH. pH is a very important parameter affecting the effectiveness of the chemical disintegration of sludge. This requires using additional reagents to acidify the sludge at the beginning as well as to neutralize after disintegration, prior to dewatering. Fenton's process is usually performed under acidic conditions (initial pH about 3.0) because they are optimal for the effective formation of hydroxyl radicals. The use of Fenton's reagent allows decreasing the capillary suction time as well as the specific resistance to filtration by even more than 90% ( Zhen et al., 2014 Lu et al., 2003 Buyukkamaci, 2004). If dewatering properties of sludge are the aim of disintegration, usually short process times are used that are not longer than 90 min. For example, if we use Fenton's reagent they are in the range of 100–6000 mg of H 2O 2 per 1 L of sludge or up to 50,000 mgH 2O 2 per 1 g of d.m. When improvement of dewatering of sewage sludge is planned, high doses of reagents are usually high. They depend on the kind of sludge and the effect that should be achieved. The doses of reagents used for sludge stabilization and disintegration vary a lot. Increasing the hydrolysis rate of excess sludge Improving susceptibility of thickened sludge to dewatering, increasing the degree of organic compound degradationĮnhancing the rate of aerobic sludge stabilization Improving susceptibility of excess sludge to dewatering
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