APPLICATIONS: THE BIG THREE

The three most important control parameters in the operation of a belt filter press are hydraulic loading , solids loading , and polymer dosage .

Hydraulic loading, or flow rate, is expressed in, for example, US gpm per meter of filter; thus, a 3-meter-wide belt filter press operating at 40 US gpm will process 120 US gpm, or approximately 27.2 m³/h of sludge.

Solids loading is usually expressed in pounds/hour or kg/h of dry residue per meter, e.g., 400 kg/h, to cite an illustrative value.

Polymer dosage is expressed in kg or lb of product per ton of input dry residue.

The four key parameters to monitor , in terms of reflecting the unit's performance, are the percentage of capture or recovery, the concentration of solids in the output ("cake dryness"), the consumption of wash water ("washwater"), and the wastewater discharge flow rate ("wastewater", i.e., the sum of wash water plus supernatant).

The capture percentage is calculated as the ratio of total solids at the inlet minus suspended solids in the discharge (filter supernatant plus backwash water) divided by total solids at the inlet. The solids concentration at the outlet is expressed as the dry residue per unit weight of concentrate, e.g., 17%. To allow for quick comparison with the hydraulic loading rate, expressed in US gpm per m³, it is common to express backwash water consumption and wastewater generation in the same units .

With varying performance levels, belt filter presses can essentially dewater sludge, whether thickened or not, and whether digested or not. The most important characteristics are the inlet solids concentration, the nature of the sludge, and the type of pretreatment, whether biological or chemical. Generally, it is recommended to maintain a minimum dry residue level of 4% at the inlet to the belt filter press. In cases where the filter is using more dilute sludge, the selection criterion is strictly hydraulic, for example, specifying a maximum of 50 gpm per 1-m of filter width. It should be remembered that lower inlet solids percentages will result in lower outlet concentrations and higher polymer consumption. There have been unfortunate reports of units that "never worked." On the other hand, if the inlet concentration exceeds 6%, the limiting factor is the solids load per 1-m of filter width.

The polymer dosage is optimized by considering the type and characteristics of the sludge, as well as the feed rate. The belt speed determines the dwell time in the gravity drainage zone, and the tension determines the stress exerted on the sludge layer confined between the two belts. If the solids load is disproportionate, i.e., too high, insufficient flow occurs in the gravity drainage zone, which can result in sludge extrusions both on the belt and at the edges. Similarly, excessive belt tension, in addition to causing potential extrusions, significantly shortens the belts' lifespan. The equilibrium point, or optimal operating point in terms of solids load, is achieved by balancing polymer consumption and the operating range. In other words, by increasing belt tension and polymer consumption, it is possible to increase the concentration at the outlet, e.g., from 21% to, say, 24%. As we mentioned earlier in another section, arbitrary modification can cause premature deterioration or unnecessary waste.

The operating schedule of a small plant might consist of a 7-hour work shift plus an additional hour for startup and shutdown. In larger plants, it is not unusual to have significantly longer work periods, i.e., between 15 and 23 net working hours plus the hour for setup and shutdown.