Organic sludge reduction

Introduction

Dealing with the issues of sludge disposal and removal from lagoons are major challenges for the municipal and industrial wastewater treatment sectors. Because a lagoon system does not waste sludge on a regular basis, accumulations in the ponds can substantially reduce the hydraulic retention time and ultimately compromise the effluent quality.

In recent years, the disposal possibilities for biological sludge from wastewater treatment plants and industries have been dramatically restricted. It is not or soon will not be possible for most biological waste sludges to be disposed of or used for agricultural purposes.


They must be processed in an other way such as dumping on a landfill or incineration. Furthermore, in a number of countries, restrictions on dumping on landfills will come into force for residues containing more than 10% of organic matter. Each of these restrictions will cause an increase in sludge disposal costs for both water boards and industries in addition to the doubling of sludge disposal costs that has already taken place over the last decade.

There are two ways to deal with the sludge and their removal : "the anaerobic way" & "the aerobic way".

In an unaerated pond, organic sludge is primarily biodegraded anaerobically. Anaerobic processes release noxious gases such as H2S (hydrogen sulfide) and harmful greenhouse gases such as CH4 (methane). One molecule of CH4 has 30 times more impact on the greenhouse effect than one molecule of CO2, which is produced during aerobic digestion.

In cold water temperatures digestion essentially stops. As a result, sludge accumulates at an increased rate in winter. When water temperatures increase in spring anaerobic digestion resumes. This increased activity combined with a thermal inversion is what causes significant spring odor production in unaerated or poorly aerated ponds. Anaerobic sludge digestion also reintroduces nutrients such as ammonia and phosphorous into the water column which can result in discharge compliance problems.

The "aerobic way" is based on a biological process. The basic function of a biological treatment system is to convert soluble organics to carbon dioxide, water and bacterial cells. The cells can then be separated from the purified water and disposed of in a concentrated form called "sludge". The advantage of biological treatment is that it can reduce soluble organics in large volumes of water far more economically than any other type of chemical or physical / chemical process.

Solution

PNW solutions provide an effective organic sludge removal system based on the combination of ADS aerationand Bacta-Pur® bioaugmentation. ADS aeration delivers oxygen and continual mixing to the sludge water interface, which results in aerobic biodegradation of low-density (organic) sludge. The production of CH4 and the liberation of nutrients into the water column are limited by the oxygenation of the sludge and water.

In order to reduce rates of organic sludge accumulation, a well designed ADS aerated pond is generally equiped with a high performance Bacta-Pur® system. Bacta-Pur® is continuously injected directly into the sludge layer In order to maximize reductions of existing or "old" sludge.

The construction of expensive drying beds or the costs associated with mechanical equipment and trucking are not required when sludge reduction using ADS and Bacta-Pur® is installed. Because sludge processing or storage sites are not required, no costly permits are necessary.

Removal of inorganic material such as rags or plastics must be performed mechanically through either wet dredging or dry excavation.

When mechanical desludging is required, the diffusers are simply disconnected from the header and coiled or removed, leaving the lagoon bottom free of mechanical equipment for easy desludging access. After sludge removal the lines are placed back in the cell and reconnected to the header.

Performance

Approximately 75% of the total system sludge accumulation occurs in the first treatment cell of an ADS aerated lagoon system. A typical system will convert (biodegrade) 75% of the organic solids in the system into carbon dioxide and water. PNW ADS aerated lagoons are typically designed to handle 20 years of sludge accumulation without mechanical removal.

With a combined ADS and Bacta-Pur® system, "old" organic sludge (15 years) in previously anaerobic ponds, is typically reduced between 50 and 80%.