System and method for treating wastewater and resulting primary and biological sludge

A method of treating wastewater and resulting sludge comprising:

• subjecting the wastewater to primary treatment and producing primary sludge;
• after subjecting the wastewater to primary treatment, subjecting the wastewater to biological treatment and producing biological sludge;
• directing the biological sludge to a thermal hydrolysis unit and subjecting the biological sludge to a thermal hydrolysis process and producing thermally hydrolyzed biological sludge;
• bypassing the thermal hydrolysis unit with the primary sludge and not subjecting the primary sludge to the thermal hydrolysis process;
• wherein the temperature of the non-thermally hydrolyzed primary sludge is lower than the temperature of the thermally hydrolyzed biological sludge;
• after thermally hydrolyzing the biological sludge, mixing the thermally hydrolyzed biological sludge with the non-thermally hydrolyzed primary sludge to form a sludge mixture and transferring sensible heat from the thermally hydrolyzed biological sludge to the non-thermally hydrolyzed primary sludge in order to raise the temperature of the non-thermally hydrolyzed sludge and lower the temperature of the thermally hydrolyzed biological sludge;
• after mixing the thermally hydrolyzed biological sludge with the non-thermally hydrolyzed primary sludge, directing the sludge mixture to a pasteurization unit and pasteurizing the mixture of thermally hydrolyzed biological sludge and non-thermally hydrolyzed primary sludge in the pasteurization unit at a temperature of at least 70° C. for a period of at least 20 minutes;
• and
• after pasteurizing the mixture of sludge, transferring the mixture of thermally hydrolyzed biological sludge and non-thermally hydrolyzed primary sludge from the pasteurization unit to an anaerobic digester and subjecting the mixture of sludge to anaerobic digestion.

A system for treating wastewater and resulting sludge comprising:

• a primary treatment unit for treating wastewater and producing primary sludge;
• a biological wastewater treatment unit disposed downstream of the primary treatment unit and wherein the biological wastewater treatment unit produces biological sludge;
• a thermal hydrolysis unit operatively connected in the system for receiving biological sludge and thermally hydrolyzing the biological sludge;
• a pasteurization unit disposed downstream of the thermal hydrolysis unit;
• a pressurized line leading from the thermal hydrolysis unit for conveying the thermally hydrolyzed biological sludge from the thermal hydrolysis unit and wherein the pressure within the pressurized line is greater than atmospheric pressure;
• a primary sludge feed line for conveying primary sludge and communicatively connected to the pressurized line for conveying primary sludge into the pressurized line;
• wherein the primary sludge feed line bypasses the thermal hydrolysis unit resulting in the primary sludge not being subjected to thermal hydrolysis;
• a pump for pumping primary sludge through the primary sludge feed line into the pressurized line where the thermally hydrolyzed biological sludge and primary sludge are mixed to form a sludge mixture;
• wherein there is an absence of heat exchangers associated with the pressurized line such that the thermally hydrolyzed biological sludge being conveyed in the pressurized line is not cooled by heat exchangers and wherein the pressurized line is configured such that substantially all cooling of the thermally hydrolyzed biological sludge is accomplished by transferring sensible heat from the thermally hydrolyzed biological sludge to the primary sludge in the pressurized line;
• wherein the system is operative to transfer the sludge mixture from the pressurized line to the pasteurization unit where both the thermally hydrolyzed biological sludge and primary sludge undergo a pasteurization process;
• and
• an anaerobic digester disposed downstream from the pasteurization unit for receiving the mixture of sludge and performing an anaerobic digestion process on the mixture of sludge.