Solids and wastes are collected as sludge in a variety of water treatment processes. These could include excess sludge from different sources such as the underflow of clarifiers. Produced sludge in different stages of water/wastewater treatment facilities should be dewatered and disposed of properly.
Sludge is a difficult and special fluid to handle. It needs special equipment for handling, transferring and processing. Further, sludge disposal requires great care as there are strict environmental rules to follow. However, sludge treatment and management systems are often overlooked. This has led to plant shutdowns, financial loss, penalties and operational problems. This article discusses sludge treatment and management systems for water and wastewater treatment units.
Sludge Tank System
The first stage after the collection of sludge is usually a sludge holding tank system, which will feed the sludge to the next stage of treatment. A sludge tank system is typically used for thickening and sludge flow equalization. There should be sludge holding tank recirculation pumps and sludge transfer pumps.
The sludge tank usually has a capacity of one day of storage. Sometimes, however, the sludge tank system is designed for several days of storage. For large plants, it may hold just half a day of sludge production. Usually two sludge tanks (“1+1” configuration) are used. Often these operate on a fill and draw basis. Ideally, each tank should be provided with a minimum of two sludge pumps (one operating and one standby), which can be used for circulating and mixing of contents. Usually, if a plant produces different sludges — such as primary sludge and secondary sludge — they are combined and treated together.
Dewatering equipment is needed to reduce the volume of liquid in produced sludge. Sludge dewatering can be carried out by different systems such as a filter press, decanter centrifuge and others. Most often a filter press system is used, although other options for dewatering are available. A filter press system typically includes accessories such as feed pumps, conditioning tanks/mixers, sludge transfer pumps and mixing pumps. All of them require auxiliary systems, controls, instrumentation, interconnecting piping, etc.
A filter press is a tool used in solid/liquid separation that uses the principle of pressure drive. This offers a simple, reliable method of compressing and dewatering sludge into cakes of 25–75% total concentration. The filter press is usually used in fixed-volume and batch operations, which means that the operation should be stopped to discharge the filter cake before the next batch can be started. There have been other types, concepts and models as well.
The plate and frame filter press is a common configuration. It consists of many plates and frames assembled alternately with the support of a pair of rails. Plates are often of the non-gasketed type. They come complete with filter cloths factory installed.
The electric and hydraulic opening and closing system (actuator) of a filter press needs careful configuration/detail design for reliability and proper operation. Such a system typically includes a double acting hydraulic cylinder and one or two hydraulic power packs. One reliable unit or two power pack units (“1+1” configuration) can be used. The system should automatically compensate for any thermal expansion or contraction of the plate stack as well as maintain the proper clamping force throughout the process cycle.
Pumps for Sludge Services
Pumps for sludge services are special pumps. Positive displacement pumps have been used in sludge and dense liquid services. Some positive displacement pumps have presented poor performance and low reliability. Therefore, special care is needed to select the proper positive displacement pumps for sludge services. A proper pump for a sludge service should have successful operating references in similar sludge services.
Material selection and pump systems should be suitable for a typical pH range of 5 to 12, at a temperature range of 5°C to 70°C, and passing large solid sizes (around 50 mm). The capacity and turndown capability of the pumps should be sufficient to handle the sludge produced by the plant, based on the plant flows, and appropriate for the sludge dewatering system.
These pumps should be capable of operating dry for a period of time without damage. Easy access and maintenance are important as pumps in this service are usually subjected to excessive wear and damages. Often pumps incorporate repair-in-place provisions that allow them to be serviced without removal or disturbing the suction and discharge piping. There have always been risks of blockage and clogging. Potential blockages can be removed through proper provisions such as an access cover. Any pump seal or packing in sludge services will cause operational and reliability issues. Therefore, their elimination is a great step toward smooth, trouble-free, long-term operation.
Progressive Cavity Pumps
Progressive cavity pumps (PCP) consist of a helical rotor, which turns inside a helical stator. The rotor, fabricated from a suitable material (stainless steel, alloy steel, etc.), is machined to a high degree of precision, and the stator is molded in a resilient elastomer or similar. As PCP pumps are well-known and explained in other articles, they are not discussed further here.
Double Disc Pumps
A double disc pump is a positive displacement pump that uses a trunnion (elastomer), a disc and a connecting rod to force sludge from the suction side to the discharge side of the pump by creating a vacuum inside the pump body. The suction and discharge valve chambers (with the check valves) help to manage higher-than-normal concentrations of grit and solids in pumped sludge.
Double disc pumps combine the performance features of a positive displacement pump and the principle of induced flow to provide good operation and superior versatility in sludge services. These pumps use a unique principle of operation in which the discs perform the duties of both diaphragm and valve, providing a double-acting, non-clogging, pumping action. This non-clogging positive displacement pumping action is important for sludge services. In these pumps, through an arrangement of connecting rods and a camshaft, a reciprocating action of the discs is created, forming a large cavity between the discs. This cavity is filled and exhausted in a continuous flow of sludge.
The large valve-like discs mean that solids and rags can be handled without loss of pumping action. These discs have large seating areas that provide for low internal velocities, extending the pump wear life on abrasive sludges and slurries.
In a typical double disc pump, sludge flows into the suction side where the check valve (usually a ball check valve) helps to break up and manage solids, particles and rags. The sludge then meets the first disc and trunnion, which creates a vacuum-like suction inside the pump body and forces it to the discharge side. The suction and discharge discs are most often universal and interchangeable with each other to increase the commonality of spare parts.
Connecting rods should be fabricated from a suitable hard alloy steel. The drive shaft and the drive system should be capable of withstanding a dead head situation. The shaft is usually mounted on four self-aligning, sealed bearings.
Old-fashioned double disc pumps have presented low reliability and they were maintenance intensive. Some of them reached high speeds of around 500 rpm, which presented faster wear of components. Modern double disc pumps, however, have presented reliable and trouble-free operation over the last decade. They are low speed pumps usually under 120 rpm. Low speed combined with reliability-focused models have led to longer lasting parts and less wear. In these modern pumps, the disc never contacts the pump body and sludge travels along a contoured velocity channel, keeping solids beyond reach of trunnion and disc. Disc and trunnion can be replaced during maintenance without removal of piping.
Sealing of the pump fluid chamber is usually achieved by flexible trunnions. In other words, the sludge chamber is usually sealed with flexible trunnions, which eliminates packings, mechanical seals and requires no flushing water or other forms of flushing/lubrication.
Poorly designed sludge treatment and management systems can lead to undesirable consequences, such as plant shutdowns, financial loss, penalties and operational problems. Familiarity with the basic sludge management and treatment components outlined here will help operators ensure these systems are not overlooked. WT
About the Author: Amin Almasi is a lead mechanical engineer in Australia. He is chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical) and IMechE (CEng MIMechE) in addition to a M.Sc. and B.Sc. in mechanical engineering and RPEQ (Registered Professional Engineer in Queensland). He specializes in mechanical equipment and machineries including pumps, condition monitoring, reliability, as well as power generation, water treatment, and others. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 150 papers and articles dealing with rotating equipment, condition monitoring, power generation, water treatment, material handling and reliability.