Pathogens

Large numbers of helminth eggs were found in both raw sludge and sludge digested under aerobic and anaerobic mesophilic conditions. Helminths are common parasitic worms that include cestodes (tapeworms), nematodes (roundworms or ascariasis, and whipworms or Trichuris trichura), and trematodes (flukes). Nematodes form the main mass of eggs.

The life cycle of nematodes is typical of helminths. Larvae bore through the intestinal wall of a host, either human or animal, enter the bloodstream, and are carried to the lungs, from which they are coughed up into the mouth and returned to the intestine, where they develop into adult worms. Humans are host to about 50 species of roundworms and, in fact, more than a third of the world's population suffers from diseases caused by roundworms (Goodman et al., 1986).

The number of nematode eggs in 1 kg of primary sludge, activated sludge, and the mixture digested in aerobic or anaerobic mesophilic conditions reached several hundreds; in mechanically dewatered sludge the number reached several thousands. A portion of the helminth eggs perishes during drying on drying beds and during storage, but a large part survives for a long time, and they can be transformed into larvae, which can survive for about five years in drying beds (Turovskiy, 1988). During the experiments, special little bags with helminth eggs were incorporated into the sludge, which allowed a precise determination of the quantity of damaged eggs resulting from the treatment. It was determined that helminth eggs are destroyed within 2 hours by heating at 50°C, within several minutes at 60°C, and within several seconds at 70°C.

After mechanically dewatering sludge and heating it to 60°C, inoculation on Wilson Bleaur or Ploskiryov medium, on media with different inhibitors, or on Miller, Kaufman, and other media revealed no presence of the intestinal typhoid group of bacteria. The studies showed that because of the extreme changeability of the colon bacillus that was revealed in the process of reactivation, there should be no fear of livability or virulence of pathogenic microbes during utilization of dewatered, heated sludge.

During aerobic anaerobic digestion, helminth eggs survive for a long time. In thermophilic digestion, additional equipment for disinfection may not be necessary. For predisinfection of mesophilic digestion, single-, two-, or multistage heat exchangers may be used for heating the sludge to 60°C. The temperature in the process that produces class A sludge that meets the fecal coliform requirements has to be maintained at either 53°C for 5 days, 55°C for 3 days, or 70°C for 30 minutes (Lue-Hing et al., 1998). These temperatures meet not only the fecal coliform requirements, but also density requirements for viruses and viable helminth ova.

Russian Construction Standards and Regulations for disinfecting sludge allow the following methods to be used:

• Thermal: heating, drying, incineration

• Biothermal: composting

• Chemical treatment

• Biological: extermination of microorganisms by unicellular fungi and by soil plants

For thermal drying, the standards require heating the entire mass of sludge to not less than 60°C. The moisture in the thermally dried sludge is recommended to be not less than 25% because evaporation of bound water requires excessive energy to achieve a moisture content below 25%, sludge with less than 25% moisture becomes hygroscopic and is able to absorb water from the air, and dust is formed when drying sludge to less than 25% moisture.

Raw sludge treated with hydrated lime to increase its pH to greater than 10 loses its odor, coliform and enterococcus are suppressed, but there is no significant effect on helminth eggs. Hydrated lime increases temperature during hydration; 1 mol (56 g) of calcium oxide generates 65 kJ of heat:

During the process of hydrating 1 kg of chemically pure lime (100% CaO), 1152 kJ of heat is produced, requiring 320 g of water.

The quantity of heat in kilojoules that is required for heating sludge by lime treatment may be determined by the formula

Q = (MSLCSL + MlCL) AT (10.6)

where

Q

= quantity of heat, kJ

Msl

= sludge mass, kg

CSL

= specific heat of sludge, kJ/kg • °C

Ml

= lime mass, kg

Cl

= specific heat of lime, kJ/kg • °C = 0.92

AT

= difference in temperature that is necessary to heat the sludge

and prime temperature of the sludge

The specific heat of sludge may be calculated as follows:

where

1.8 = specific heat dry sludge with a moisture of 5 to 10%, kJ/kg • °C WSL = sludge moisture, %

The heat in kilojoules produced by lime hydration, taking into consideration its activity by CaO, is

where

Qr = heat, kJ A = lime activity, % Ml = lime mass, kg

The increase in sludge temperature caused by a predetermined dose of lime may be calculated preliminarily by using the equation of material balance:

Given the required difference in temperatures, the mass of lime necessary for sludge treatment may be calculated as follows:

The formula for calculating sludge moisture after lime addition, assuming complete hydration of lime, is

where WK is the sludge moisture after lime addition, %.

The amount of lime (ML) in kilograms required to decrease the moisture of 1 metric ton of sludge to the required level may be determined as

The equations above may be used only for approximate calculations, because in reality, lime is not completely hydrated because of the internal structure of lime (which is not homogeneous by the size and quality of particles) and the presence of bound water in sludge with different forms of binding between solid particles and water. Lime consumption depends very strongly on the sludge moisture and therefore on the volume of the treated sludge. Therefore, dewatering of sludge is highly recommended.

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