Vacuum Filters

Vacuum filters remove suspended solids from water and wastewater by drawing liquid through a porous filter medium using applied vacuum pressure. The process creates a pressure differential that forces water through the filter media while retaining particles on the surface, forming a filter cake. Typical municipal installations achieve 85-95% solids removal efficiency with feed concentrations ranging from 100-5,000 mg/L TSS. The primary limitation is frequent backwashing requirements and potential blinding of filter media with fine particles, making them less suitable for highly variable influent conditions without adequate pretreatment.

• Biosolids Dewatering (Primary Application) - Vacuum filters serve as the final dewatering step in biosolids processing trains at 2-25 MGD plants. Following gravity thickening and anaerobic digestion, conditioned sludge (typically 3-6% solids) feeds to the vacuum filter, producing cake at 18-25% solids
• Filter Backwash Solids Recovery - At plants with rapid sand or multimedia filters, vacuum filters process backwash settling tank underflow containing filter media fines and captured particles. This application typically handles 0.5-2% solids concentration
• Scum Processing - Primary and secondary clarifier scum, after screening and grinding, requires dewatering before disposal. Vacuum filters effectively process this high-grease content material, achieving 15-20% solids

Daily Operations - Operators monitor vacuum gauge readings (target 18-22 inches Hg), drum rotation speed, and feed flow rates hourly. Cake thickness observation through sight ports indicates proper operation - thin cake suggests high vacuum or fast rotation, while thick cake may indicate media blinding. Feed rate adjustments maintain consistent 4-8 hour filtration cycles based on solids loading.

​

Maintenance - Weekly filter media inspection identifies tears or blinding requiring cleaning or replacement. Monthly vacuum system maintenance includes moisture trap drainage and vacuum pump oil changes. Quarterly drum bearing lubrication and annual media replacement represent major maintenance items. Confined space entry procedures apply for internal drum inspection, requiring gas monitoring and attendant coverage.

​

Troubleshooting - Declining vacuum indicates media blinding, vacuum system leaks, or pump wear - typical service life spans 8-12 years for properly maintained units. Uneven cake formation suggests damaged media sections or improper blade adjustment. Poor cake release often indicates worn doctor blades or inadequate vacuum, while excessive filtrate turbidity signals media breakthrough requiring immediate replacement to prevent downstream process upset.

• Rotating Drum Assembly - The perforated steel drum (6-12 feet diameter, 4-16 feet length) provides filtration surface area ranging from 100-1,500 sq ft for municipal applications. Drum rotation speed (0.1-2 RPM) controls cake thickness and filtration time
• Vacuum System - Rotary positive displacement blowers or liquid ring vacuum pumps maintain 15-25 inches Hg vacuum. Sizing ranges from 100-2,000 CFM capacity based on drum surface area and desired filtration rate
• Filter Media - Synthetic filter cloths (polypropylene or polyester) with 10-100 micron openings attach to drum segments. Media selection depends on particle size distribution and chemical compatibility
• Cake Discharge System - Doctor blades or scraper assemblies remove dewatered cake at 1/8 to 1/2 inch thickness. Pneumatic or hydraulic positioning systems maintain proper blade contact pressure

• Vacuum Level: 15-25 inches Hg (typical municipal operation), with capability to 27-28 inches Hg maximum
• Solids Loading Rate: 2-8 lbs/hr/ft² filter area for municipal sludges. Primary sludge typically 4-6 lbs/hr/ft², waste activated sludge 2-4 lbs/hr/ft², digested sludge 6-8 lbs/hr/ft²
• Cake Solids Content: 18-35% depending on sludge type. Primary sludge achieves 25-35%, WAS typically 18-25%, digested sludge 20-30%
• Filter Yield: 0.5-4.0 lbs cake solids/ft²/cycle. Varies with sludge characteristics and precoat thickness
• Drum Speed: 0.1-2.0 rpm variable speed control. Slower speeds increase cake thickness and solids content
• Precoat Application Rate: 0.1-0.3 lbs/ft² diatomaceous earth or similar filter aid
• Filtrate Turbidity: <50 NTU typical, <20 NTU achievable with proper precoat
• Wash Water Requirements: 2-5 gpm/ft² drum circumference for cake discharge and cloth cleaning
• Submergence: 25-40% of drum diameter in sludge bath for optimal performance

• What solids loading rate should be specified for the expected sludge characteristics? Primary sludge allows 4-6 lbs/hr/ft², while WAS requires 2-4 lbs/hr/ft². Overloading causes poor cake formation and blinding; underloading wastes filter capacity and increases unit costs
• Should the system include precoat capability and automatic precoat application? Precoat improves filtrate quality and prevents cloth blinding but adds $50,000-100,000 to system cost and requires ongoing consumable costs of $0.10-0.30/lb dry solids
• What level of automation is required for unattended operation? Basic manual systems cost 30-40% less but require constant operator attention. Fully automated systems with PLC control, automatic precoat, and CIP cycles enable 8-16 hour unattended operation but increase complexity
• How will polymer conditioning be integrated? Inline polymer addition improves performance but requires precise dosing (2-8 lbs/ton dry solids). Poor polymer mixing or overdosing can blind filter media and reduce capacity by 50% or more

• MasterFormat 46 13 16 - Vacuum Filters (Water and Wastewater Treatment Equipment)
• Secondary: 40 30 00 - Process Interconnections for polymer feed systems and filtrate handling piping

• Material/Equipment Verification: Verify filter media specifications and replacement availability, confirm vacuum pump sizing and redundancy requirements, check filtrate quality guarantees against permit limits
• Installation Requirements: Substantial concrete foundations for rotating equipment, overhead crane access for media replacement, vacuum piping with proper slope and cleanouts
• Field Challenges: Alignment of rotating drums critical for even cake formation, filtrate piping must handle solids carryover
• Coordination Issues: 16-20 week lead times typical for custom municipal units

• Komline-Sanderson - Rotary drum filters, Model RVF series for 0.1-50 MGD applications
• Andritz - BELT-THICKENER vacuum belt filters, popular for biosolids dewatering
• WesTech Engineering - Rotary vacuum precoat filters, Model RVF for water treatment
• Parkson Corporation - DynaSand vacuum backwash systems for filtration applications

• Belt Filter Presses - Better dewatering for biosolids (18-25% vs 15-20% solids), higher capital cost but lower polymer usage
• Centrifuges - Smaller footprint, higher maintenance, typically 20-30% more expensive but handle variable loads better
• Screw Presses - Lower energy consumption, simpler operation, emerging technology for smaller plants under 5 MGD with competitive lifecycle costs

Establish spare parts inventory early - filter media and vacuum pump seals are consumables with 6-month to 2-year replacement cycles. Negotiate training packages including hands-on troubleshooting. Consider performance guarantees tied to specific feed characteristics rather than general ranges. Build relationships with local manufacturer service technicians before startup - their field experience often exceeds what's in manuals for optimizing cake thickness and wash cycles.