Sludge is the only material in this library where the pre-treatment has to be written into the selection rationale itself. Municipal and industrial sludge commonly arrives at around 80 percent moisture, and most of that water is bound; putting it straight into a hot dryer is both expensive and ineffective, because the water held by the silt and the organics has to be squeezed out mechanically first, by screw press, centrifuge or plate-and-frame filter. As a rule of thumb, bring the moisture down to around 50 percent before any heat is applied, and the choice at that point is the single most important economic decision in the whole process.
Sludge is the only material in this library where the pre-treatment has to be written into the selection rationale itself. Municipal and industrial sludge commonly arrives at around 80 percent moisture, and most of that water is bound; putting it straight into a hot dryer is both expensive and ineffective, because the water held by the silt and the organics has to be squeezed out mechanically first, by screw press, centrifuge or plate-and-frame filter. As a rule of thumb, bring the moisture down to around 50 percent before any heat is applied, and the choice at that point is the single most important economic decision in the whole process. Our configuration is the rotary drum: sludge needs continuous tumbling to break it up, and it cakes into hard lumps inside the drum quite readily, so the flight pattern has to be matched to the stickiness and a disintegrator may be needed at the feed end. Batch drying is the alternative for small throughput with variable feed. Two points have to be confirmed early. First, the exhaust carries ammonia and odour, and the condensing and deodorising sections are the part most often left out of the calculation. Second, sludge may contain heavy metals, so the end use and the discharge standard have to be settled against the customer's actual situation.