Mechanical press

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Overview
Bucher Unipektin piston presses are mechanical sludge dewatering machines designed to remove water to the limits of physical dehydrability, making them among the most effective dewatering technologies. Higher dry solids can be achieved when the press is combined with sludge disintegration or hydrolysis. Implementations report a technical service life exceeding 40 years. The presses are manufactured by Bucher Unipektin AG (Switzerland).

Principle of operation
The press core components are a cylinder and a rotating piston. Grooved polyurethane drains located between piston and cylinder discharge filtrate to the sewage system. Sludge pre-mixed with flocculant is fed into the cylinder by a screw pump, filling the space between drains. Cylinder diameter and the number of drains (filtration surface) determine throughput. The piston rotates at approximately 6 revolutions per minute and subjects the sludge to cyclic compression and expansion; effluent is expelled through grooved polyurethane cores covered with filter fabric.

Peripherals and controls
Feed is provided by a screw pump and flocculant is dosed in a dynamic mixer. Each installation includes microwave-based dry solids measurement to dose polymer according to dry mass and flow, reducing overdosing. Sludge and flocculant lines are fitted with flow meters. The press is supplied with an integrated power and control system that can interface with the plant main switchboard. Continuous monitoring of average sludge dry solids and a control panel deliver real-time and archived data and enable remote/local control.

Operational aspects
Dehydration results – pilot and full-scale applications typically deliver 45–50% dry solids; select advanced combinations report up to ~70% DM, levels not achievable with conventional belt presses or centrifuges.
Energy consumption – hydraulic operation ensures low electrical consumption; indicative installation values are approximately 1.5–2 Wh/kg H2O.
Chemical use – flocculant/coagulant consumption is comparable to other technologies, typically 6–16 kg active substance (AS) per tonne DM of sludge; precise dosing is determined during pilot trials or start-up.
Effluent quality – the press yields very clean filtrate; suspended solids in effluent typically do not exceed 100 mg/l.
Odours – the press remains sealed during filling, compression and expansion cycles and only opens during unloading; dewatered sludge has a loose granular texture with a mild earthy odour.
Noise – measured noise emissions are below 85 dB during operation.

Strengths and weaknesses

• Dewatering to the limits of physical possibility
• Technical service life exceeding 40 years
• Low energy consumption (approx. 1.5–2 Wh/kg H2O)
• Minimal odour emissions due to closed system
• High effluent quality (suspended solids ≤ ~100 mg/l)
• Process flexibility
• Process control via continuous dry solids measurement
• Automatic filter fabric cleaning by countercurrent air flow during piston retraction
• Simple maintenance and comparatively low repair costs

Main disadvantages are machine footprint and relatively high capital cost; operation requires qualified personnel, a stable flocculation process and a feed above approx. 2% solids. Dewatered sludge is loose granules, so screw conveyors should operate with a filling factor not exceeding ~30%.

Caractéristiques / spécifications techniques

• Manufacturer: Bucher Unipektin AG (Switzerland)
• Product type: Bucher piston press
• Main elements: piston and cylinder with grooved polyurethane drains and filter fabric
• Piston rotation speed: approx. 6 revolutions per minute
• Dewatering performance: typically 45–50% dry solids; up to ~70% with some advanced technologies
• Energy consumption (installation indicator): approx. 1.5–2 Wh/kg H2O
• Flocculant/coagulant consumption: typically 6–16 kg AS per tonne DM (to be confirmed by pilot/start-up)
• Effluent suspended solids: typically ≤ 100 mg/l
• Noise level: below 85 dB in operation
• Technical service life: over 40 years (typical)
• Required feed solids: above approx. 2% (stable flocculation needed)
• Screw conveyor filling factor recommendation: not to exceed ~30%
• Peripherals: screw feed pump, dynamic flocculant mixer, microwave dry solids measurement, flow meters for sludge and flocculant, integrated power and control panel with data logging