Belt Filter Press
Belt filter presses consist of two endless belts that travel over a series of rollers assembled on a galvanized steel frame. Sludge is conditioned with polymer and allowed to dewater in a drainage area before it is fed into the area between the two belts. As sludge travels between the belts, it is pressed between perforated and non-perforated rollers. This causes water to be forced from the sludge and out of the belts where it is collected and returned to the treatment plant. The sludge cake is scraped off of the belts by the doctor blade and carried away on a conveyor while the two endless belts are washed to prevent plugging.
The factors that affect belt filter presses are as follows:
1. Sludge Type. Belt presses are not suitable to all types of sludges. Undigested waste activated sludge generally lacks the qualities necessary for dewatering with a belt press. Undigested WAS is often simply squeezed out from between the belts without dewatering. However, if properly digested, WAS can be successfully dewatered using belt presses.
2. Sludge Conditioning. Sludge conditioning prior to passing through the belt press is probably the most critical factor to successful dewatering. Cationic polymers are generally used for conditioning of sludge. Polymer dosages must be optimized to ensure optimal dewatering. A consistent sludge feed is imperative in order to optimize polymer conditioning.
3. Belt Tension Pressure. The pressure applied to the sludge can be increased or decreased by changing the tension roller setting. The belts should be neither too tight nor too loose. Experience will demonstrate the best setting.
4. Belt Speed. The speed at which the belt can be operated depends upon the sludge flow rate and the concentration of influent sludge. The belt speed must be fast enough to spread the sludge over a sufficient area of the belt so that drainage can occur. If not, excess water will be carried along with the sludge being pressed between the belts and the result will be washout, which causes the effluent water quality to suffer. The belt speed should be as slow as possible and yet fast enough to avoid washout.
5. Belt Type. A variety of belt materials are available such as nylon and polypropylene, each with various porosity. Higher porosity belts drain fast but leave a poor quality effluent. Low porosity belts may bind or plug causing frequent washouts. To preserve the life of a belt, belt-cleaning equipment should be kept in good working order.
Belt filter presses generally require close operator attention to attain consistent results. Problems that arise are most commonly associated with feed sludge quality changes and less than optimal polymer dosages, (which are related).
Typical Belt Filter Press Performance:
Sludge Type: Primary
Polymer, lbs/ton: 4-8
Cake, % TS: 25 – 35
Hydraulic Load, gpm/ft: 10 – 12
Sludge Type: Secondary
Polymer, lbs/ton: 9 – 20
Cake, % TS: 17 – 20
Hydraulic Load, gpm/ft: 5 – 15
Sludge Type: Digested Primary
Polymer, lbs/ton: 4 – 8
Cake, % TS: 25 – 30
Hydraulic Load, gpm/ft: 10 – 25
Sludge Type: Digested Secondary
Polymer, lbs/ton: 15 – 20
Cake, % TS: 17 – 20
Hydraulic Load, gpm/ft: 5-15