Sludge Handling and Management

Summary

​Sludge accumulates in various waste management ponds at piggeries and cattle feedlots, until eventually the build-up compromises pond capacity and it has to be removed and handled. Many existing piggery ponds are reaching the point where sludge removal is becoming a pressing issue. Additionally, the use of covers on these ponds so they can be used to generate biogas presents special difficulties with sludge removal.

FSA Consulting was engaged by Australian Pork Limited and Meat & Livestock Australia to collect, review and collate available information on current sludge removal and handling practices employed in intensive agriculture industries and other relevant areas.

Sludge is a mixture of water and solid materials (total solids (TS)). The TS component can be inorganic material (any materials such as debris, sand or rocks plus the ash component of organic wastes), slowly digestible organic material or dead microbial cell mass. The ratio of water to solids (TS content) can vary considerably. As the TS content increases, the sludge’s characteristics and handling requirements change. The particle size and particle size distribution (PSD) can vary from very fine colloidal material to larger particles. Some particles can be cohesive (i.e. they tend to stick together) while other particles such as sand are non-cohesive. Rheology is the study of the flow of matter. This is an important feature in the design of sludge removal systems.

The term – sludge – is widely used for a range of materials. However, the handling and management options for “sludge” is heavily dependent on the TS content of the material.

In this report, the following terms have been defined.

1. Effluent. This is material with a TS content of <5%. effluent is a material that can be pumped and behaves like other newtonian fluids, e.g. water.>
2. Slurry. This is material with a TS content of 5-15%. These materials are “thick” but can flow. They behave in a non-Newtonian manner and require specialised pumping equipment.
3. Sludge. This is material with a TS content >15%. Essentially, this material is too thick to pump and must be handled with bulk mechanical methods.

The actual properties of sludges derived from different sources vary, even at the same TS content. Hence, the TS contents stated above are a general guideline for use in this report rather than a fixed rule. Care needs to be taken when reviewing other work as the definition of sludge used in the literature is highly variable.

The physical characteristics of the sludge or slurry is importantly in determining the appropriate pumping and handling methods. Particle size distribution (PSD) and bulk density are important but the rheological properties have the greatest influence. Several studies have been conducted into the rheological properties of raw and digested manure in sludge or slurry forms. Most researchers find that viscosity (i.e. resistance to pumping) increases with increasing TS content and decreases with temperature. Effluent with a TS content <2% can be pumped with centrifugal pumps. slurries with a ts content of about 5-10% ts can be pumped with various types of positive displacement pumps. sludge with ts content greater than 15% is virtually impossible to pump.>

When desludging ponds, there are three techniques depending on the operation and structure of pond, sludge physical characteristics and the frequency of desludging. Desludging can be broadly categorised into three groups:

• desludging dewatered ponds (after effluent removal)
• desludging an uncovered pond containing effluent
• desludging a covered pond.  

The report deals in detail with the approaches required to undertake these processes, and recommends further work to understand the optimal frequency and equipment design for removing sludge from covered anaerobic ponds.</2%></5%.>