Putting the squeeze on sludge
Simon-Hartley's Peter McLoughlin on maintaining process efficiency
The long-term viability of a sewage sludge dewatering system relies on the operator’s ability to consistently deliver maximum efficiency from the infrastructure, which makes maintenance and continued process optimisation vital. Only working on a machine when it breaks down will keep visible maintenance costs low, but it is a strategy which can lead to increased indirect treatment and disposal costs many times greater than the perceived savings.
Operating dewatering facilities efficiently will significantly reduce both direct and indirect costs. Direct costs include sludge throughput, polyelectrolyte dosage, energy, manpower and maintenance; indirect costs are those evolving as a consequence of the dewatering operation such as filtrate treatment and dewatered sludge treatment and/or disposal.
It is not always easy to differentiate between the types of expenditure, but both can be related to the primary dewatering equipment’s operational efficiency. For example, decreasing the amount of polymer will reduce chemical consumption but may increase final filtrate treatment charges. One direct cost, however, which can potentially reduce both direct and indirect expenditure is maintenance.
Maintenance procedures must be part of routine plant operation. Ignoring this important work for short-term gain will be repaid with higher long-term costs caused by a combination of down-time and lost production. In addition, poor maintenance can reduce plant performance, leading to substantial increases in indirect operational costs. This is often seen by Simon-Hartley engineers in dewatering equipment where incorrect machine settings or worn parts have led to chemical overdosing, inefficient dewatering performance, poor filtrate quality, or in the worst cases a combination of all three.
There are three types of maintenance: reactive, planned and performance-enhancing. Reactive maintenance is totally ineffective. Planned maintenance will suffice, but in reality sludge treatment plant inputs change considerably over time, especially in terms of pollution load. Monitoring and updating direct costs relative to the incoming sludge takes account of these variations and minimises subsequent indirect treatment costs. To account for this, Simon-Hartley believes one way of optimising the whole process is to follow the performance-enhancing maintenance route.
The company believes this approach can deliver a number of benefits:
Continually maximising plant efficiency will lead to the lowest OPEX, regardless of sludge types. In addition to regular process optimisation, performance-enhanced maintenance also allows the adoption of new machine configurations when sludge types or plant requirements change significantly. An early warning system will alert engineers of imminent changes before a plant breakdown occurs. When sludge types change from the original specification, Simon-Hartley’s machines can be upgraded with alternative roller configurations, polymers or filtration media. The extra costs of these upgrades should be recovered quickly as performance is improved and OPEX decreases.
One of the most common problems with dewatering performance is polymer dosing. With too little polymer the filtrate solids and final cake performance suffer. If too much is added the belt often blinds and drainage deteriorates, reducing throughput and lowering sludge cake solids.
To compensate, operators often increase belt speed which further reduces the time the sludge spends under pressure in the belt press, causing the cake solids to decrease to an unacceptable level.
In one such case engineers recommended the operator cut the dosing rate by half, allowing the belt speed to be reduced from 3.5m/s to 1.4m/s. As a result the water company enjoyed £31,200pa savings by cutting polymer usage and £29,300 in cake savings due to reduced belt speed.
Cake solids are directly related to belt tension and time spent under pressure. However, applying too much tension can have an adverse effect on both the roller bearings and drive mechanism. Replacing the standard belt drive with a high-torque version allows tension to be increased while maintaining polymer-dosing rates. In one instance belt tension was increased from 22bar to 28bar, as a result the dryness of the cake increased from 19.4%w/w to 21.7%w/w. Filtrate flow-rate also increased by almost 1 l/hr and annual cake savings of £18,350 were achieved.
Wearing a belt
Filtration belts wear from the day they are fitted. Treat them gently and they will give long service, but if mistreated they soon wear out. As belts wear, drainage of sludge is restricted and there is a tendency for operators to increase the speed to compensate for the decrease in throughput. Subsequently, cake solids tend to fall. Fitting a new belt and reducing speed has the effect of increasing drainage time and hence improving cake solid content, in one case achieving cake savings of £18,100pa.
Sludge characteristics and operating parameters will change over the many years a dewatering system is in service. Simon-Hartley contends the most cost-effective way to deal with this is to upgrade the existing plant. A belt press, for instance, can easily change the sludge drainage and be fitted with new shear roller configurations.
More rollers of decreasing diameter can be added to increase the time under pressure and the shear pressure exerted on the sludge. More water is expelled, resulting in a drier cake. The cake can be improved further with the addition of a nip roller to push against the outside of the final shear roller ensuring maximum water extraction.
The drier cake is extremely friable and can be safely stacked ready for land spreading. The addition of a nip roller on one site increased dryness from 18.4%w/w to 22.6%w/w and effected cake savings in excess of £34,000pa.
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