Comminutor

In-line mechanical grinder that shreds solids into fine particles instead of removing them, reducing downstream clogging and pumping issues.

Overview

Comminutors are rotating cutting devices installed in the wastewater flow channel that macerate large solids into smaller pieces (typically <6-10mm). Unlike screens which remove solids for disposal, comminutors keep shredded material in the flow, making downstream pumping easier and reducing screenings handling requirements.

When to Use

• Pumping stations (protect pumps from clogging)
• Small treatment plants without screening facilities
• Industrial sites with fibrous materials
• Retrofit of plants with pump problems
• Upstream of grinders or macerators
• When screenings disposal is problematic

Key Characteristics

• Reduces solid size without removal
• Protects pumps from clogging/damage
• No screenings to handle or dispose
• Increases downstream solids load
• Requires regular maintenance (blade wear)
• Not suitable as sole pretreatment for modern plants

Operating Modes

Basic Mode

• Fixed size reduction (particles to <10mm)
• Simplified power calculation
• Constant rotational speed

Advanced Mode

• Particle size distribution modeling
• Variable speed control
• Power based on solids loading
• Clog detection and reversal

Mathematical Model

Governing Equations

Size Reduction

$$d_{out} = d_{in} \times R_{reduction}$$

Where:

• $d_{out}$ = output particle size (mm)
• $d_{in}$ = input particle size (mm)
• $R_{reduction}$ = reduction ratio (typically 0.1-0.2)

Typical performance: Reduces 90% of particles to <6-10mm

Power Requirement

$$P = P_0 + k \times Q \times SS$$

Where:

• $P$ = power consumption (kW)
• $P_0$ = no-load power (0.5-2 kW depending on size)
• $k$ = specific energy (0.002-0.005 kWh/kg solids)
• $Q$ = flow rate (m³/h)
• $SS$ = suspended solids concentration (kg/m³)

Headloss

$$h_L = K \times \frac{v^2}{2g}$$

Where:

• $h_L$ = headloss through unit (m)
• $K$ = loss coefficient (1.5-3.0)
• $v$ = approach velocity (m/s)
• $g$ = 9.81 m/s²

Typical: 50-150mm headloss when clean

Torque Requirement

$$T = F \times r$$

Where:

• $T$ = torque (N·m)
• $F$ = cutting force (N, depends on material)
• $r$ = cutter radius (m)

Design Parameters

Input Parameters

Parameter	Symbol	Unit	Range	Default	Description
Flow rate	Q	m³/h	10-5000	500	Design flow
Channel width	W	m	0.3-2.0	0.8	Channel width
Rotor diameter	D	m	0.2-0.9	0.5	Cutter diameter
Rotational speed	N	rpm	30-150	60	Cutter rotation
Bar spacing	s	mm	6-25	10	Clearance opening
Number of cutters	n	-	2-8	4	Cutting edges

Output Parameters

Parameter	Unit	Description
Particle size (max)	mm	90th percentile output size
Size reduction	%	Fraction of large particles reduced
Power consumption	kW	Electrical demand
Headloss	m	Pressure drop
Maintenance interval	hours	Operating hours between service

Configuration Guide

Step-by-Step Setup

1. Select Unit Size

   • Based on peak flow capacity
   • Channel width compatibility
   • Solids loading estimate

2. Choose Comminutor Type

   • Vertical screen: Rotating cutter against fixed bars
   • Drum: Rotating perforated drum with internal cutter
   • In-line grinder: Dual-shaft cutting mechanism

3. Determine Installation Location

   • Before pumps: Protect pumps (most common)
   • After screens: Reduce screenings, supplement screening
   • Before primary treatment: Reduce grit/fiber issues

4. Configure Drive System

   • Motor size from flow and solids load
   • VFD for variable speed (optional)
   • Torque limiter to prevent damage
   • Reverse capability for clearing jams

5. Design Channel Approach

   • Smooth transitions to unit
   • Velocity 0.6-1.2 m/s
   • Bypass channel with gate (for maintenance)

6. Plan Maintenance Access

   • Lifting equipment (unit is heavy)
   • Space for cutter removal
   • Parts storage

Typical Design Values

Application	Flow (m³/h)	Rotor Diameter	Power	Output Size
Small pump station	50-200	0.3-0.4m	1-3 kW	<10mm
Medium pump station	200-800	0.5-0.6m	3-7 kW	<8mm
Large municipal	800-3000	0.7-0.9m	7-15 kW	<6mm
Industrial (fibers)	100-500	0.4-0.6m	5-10 kW	<10mm

Cutter Design

Types of Cutters:

1. Shear bars: Fixed bars with rotating cutter passing through
2. Slotted plate: Rotating cutter pushes through perforations
3. Scissor action: Counter-rotating cutters

Clearance:

• Determines maximum particle size
• Tighter (6-10mm): Better size reduction, more power, faster wear
• Wider (15-25mm): Less power, longer life, larger output particles

Performance Criteria

Size Reduction Effectiveness

Input Size	Output Size (90th %ile)	Application
>100mm (rags, debris)	<10mm	Standard municipal
50-100mm	<6mm	High-performance unit
Fibrous materials	<15mm	May string, less effective

UK Design Standards

Parameter	Value	Standard
Approach velocity	0.6-1.2 m/s	MWH
Maximum headloss	150-250mm	Typical
Redundancy	100% standby recommended	Sewers for Adoption
Safety	Lockout/tagout required	HSE

Types of Comminutors

Vertical Bar Screen Type (Most Common)

Design:

• Stationary vertical bars (screen)
• Rotating cutter drum or shaft
• Solids shredded against bars

Advantages:

• Simple, proven design
• Effective on most materials
• Easy blade replacement

Disadvantages:

• Can clog with rags, plastics
• Regular blade sharpening needed

Examples: Muffin Monster, Wemco, Franklin Miller

In-Channel Grinder

Design:

• Low-profile unit sits in channel floor
• Dual counter-rotating cutters
• Solids pulled down and shredded

Advantages:

• Minimal headloss
• Compact installation
• Good for retrofits

Disadvantages:

• More complex mechanics
• Difficult to access for maintenance

Drum/Perforated Screen Type

Design:

• Rotating perforated drum
• Internal cutter bar(s)
• Solids forced through perforations

Advantages:

• Good screening + size reduction
• Handles high flows

Disadvantages:

• Larger, more expensive
• Can blind with fats/fibers

Troubleshooting

Issue	Cause	Solution
Frequent jamming	Rags, plastics wrapping	Install upstream coarse screen
	Dull cutters	Sharpen or replace cutting edges
	Foreign objects (metal, rocks)	Remove, check for damage
	Motor undersized	Verify torque, upgrade if needed
High power consumption	Blunt cutters	Replace/sharpen cutters
	High solids load	Normal, may need larger unit
	Binding/rubbing	Check alignment, clearances
Excessive vibration	Cutter imbalance	Balance rotor, check for damage
	Bearing wear	Replace bearings
	Loose mounting	Tighten bolts, check foundation
Poor size reduction	Wide clearance	Adjust clearance, replace worn parts
	High flow velocity	Reduce flow or increase unit size
	Cutter orientation wrong	Check installation, manufacturer specs
Odor/septicity downstream	Increased retention time	Expected (more surface area)
	Organic matter shredding	Normal, not removable
Motor trips/overload	Jam condition	Clear obstruction, check thermal overload
	Torque spike	Verify torque limiter setting

Operation & Maintenance

Daily Checks

• Visual inspection of operation
• Listen for unusual noise
• Check for visible leaks
• Monitor power consumption (if instrumented)
• Verify no jamming/stalling

Weekly Maintenance

• Check cutter clearance (wear)
• Inspect for wrapped materials
• Lubricate bearings (per manufacturer)
• Clean any debris from guards/housing

Monthly Tasks

• Detailed inspection (shut down, lockout)
• Measure cutter-to-bar clearance
• Check drive belt/chain condition (if applicable)
• Inspect motor and gearbox
• Verify torque limiter setting

Quarterly Tasks

• Replace or sharpen cutting edges
• Inspect bars/screens for wear
• Check bearing condition
• Verify alignment
• Test emergency stop

Annual Tasks

• Major overhaul (bearings, seals, cutters)
• Structural inspection
• Drive system service
• Performance evaluation (particle sizing)
• Update spare parts inventory

Advantages & Disadvantages

Advantages

• No screenings to handle or dispose
• Protects downstream pumps
• Reduces clogging in pipes and tanks
• Relatively low capital cost
• Small footprint

Disadvantages

• Does NOT remove solids (increases downstream load)
• Increases BOD/COD in wastewater (releases organics)
• Not suitable for MBR or membrane systems (damages membranes)
• Regular blade replacement required
• Can increase grit abrasion
• May cause septicity (more surface area for bacteria)

Modern Practice

Comminutors in Contemporary Design

Historical:

• Common in 1960s-1980s
• Used as alternative to screening
• Pumping stations often used comminutors alone

Current Trend:

• Declining use in municipal treatment plants
• Screening preferred (removes material, better for biological treatment)
• Still used in pumping stations for pump protection
• Not acceptable as sole pretreatment for:
  • MBR systems
  • Advanced nutrient removal
  • Stringent effluent standards

Recommended Applications Today

Good Applications:

• Lift/pump stations (supplemental to screening)
• Industrial sites with specific shredding needs
• Small package plants (limited space)

Poor Applications:

• Primary treatment replacement (use screens instead)
• Upstream of membranes (particles damage membranes)
• High-performance biological systems (prefer solids removal)

Comparison with Screening

Feature	Comminutor	Screening
Solids removal	0% (shreds only)	30-80% (removes)
Downstream load	Increases	Decreases
Screenings handling	None	Required (disposal)
Pump protection	Good	Excellent (removes)
MBR/membrane suitability	Poor	Excellent
Biological treatment	Increases load	Reduces load
Capital cost	Low-medium	Medium-high
Operating cost	Medium (blades)	Low-medium (disposal)

Current Best Practice:

• Primary: Use screening (coarse + fine)
• Supplemental: Add comminutor at pump stations if needed
• Not: Comminutor as sole pretreatment

Worked Example

Given

• Pumping station: 300 m³/h peak flow
• Channel width: 0.8m
• Influent TSS: 400 mg/L
• Solids with rags, paper, organics
• Goal: Protect downstream pumps

Design

Step 1: Select Comminutor Size

From manufacturer data, choose unit rated for 400 m³/h

Step 2: Verify Channel Velocity

Assume depth at peak flow: d = 0.6m

$$v = \frac{Q}{W \times d \times 3600} = \frac{300}{0.8 \times 0.6 \times 3600} = 0.17 \text{ m/s}$$

Too low! Channel should be narrower or unit should be installed in a contraction.

Revise: Install with contraction to 0.5m width: $$v = \frac{300}{0.5 \times 0.6 \times 3600} = 0.28 \text{ m/s}$$ ✓ (acceptable)

Step 3: Estimate Power

No-load power: P₀ = 2 kW (typical for this size)

Specific energy: k = 0.003 kWh/kg

Solids loading: 300 m³/h × 0.4 kg/m³ = 120 kg/h

$$P = 2 + 0.003 \times 120 = 2.36 \text{ kW}$$

Select 5 kW motor (safety factor ~2)

Step 4: Estimate Headloss

K = 2.0 (typical)

$$h_L = 2.0 \times \frac{0.28^2}{2 \times 9.81} = 0.008 \text{ m} = 8 \text{ mm}$$

Very low (acceptable)

Step 5: Configuration

• Install upstream of pumps
• Provide bypass channel with gate
• VFD for variable speed (optional, for energy savings)
• Torque limiter: Set to 150% of normal operating torque
• Reverse function for jam clearing

Step 6: Maintenance Planning

• Inspect cutters monthly
• Replace/sharpen cutters every 3-6 months (depends on wear)
• Keep spare cutter set on hand
• Budget ~£500-1000/year for consumables

Related Units

• Coarse-Screen - Alternative (removal vs. shredding)
• Fine-Screen - Downstream screening
• Pumps - Protected by comminutor
• Grit-Chamber - May see increased abrasion with shredded solids

References

1. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8.
2. Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed.
3. Franklin Miller Inc. Comminutor Design and Application Guide.
4. JWC Environmental. Muffin Monster Technical Manual.
5. Water UK (2017). Sewers for Adoption, 8th Ed.
6. Tchobanoglous, G. et al. (2003). Wastewater Engineering: Treatment and Reuse, 4th Ed.