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The Clinton Wastewater Treatment Plant
Massachusetts Water Resources Authority


The Clinton Wastewater Treatment Plant provides advanced sewage treatment services to the town of Clinton and the Lancaster Sewer District. MWRA assumed formal operational responsibility for the Clinton plant in 1987.

Since then, MWRA has designed and constructed new primary, secondary, and advanced treatment facilities that incorporate rehabilitated portions of the existing plant with new construction.

The new facilities, designed to meet all current and projected NPDES discharge standards, were completed in 1992. A Phosphorus Reduction Facility was completed in 2017, designed to meet discharge standards in the NPDES permit issued in 2017.


The plant provides secondary treatment using an activated sludge process in combination with advanced nutrient removal, disinfection and dechlorination.

The major facilities include a headworks, primary settling tanks, digesters, sludge processing, trickling filters, aeration tanks, secondary tanks, and a phosphorus reduction facility. The plant discharges its effluent into the South Nashua River in accordance with the discharge limits of the facility's NPDES permit. Residual materials are pressed and transported to an MWRA owned landfill for disposal. Staff also perform regular monitoring of the landfill site.


Click the text in the image below for more details about the treatment plant and its processes.

Aerial of Clinton WTP

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Clinton Wastewater Treatment Plant Statistics

Influent Receiving Structures
5 Influent lift Pumps

Design Capacity (MGD) Avg - 3.01 MGD, Max - 12 MGD
Two submersible pumps (VFD): 3.5 MGD
Three screw pumps:
     Slow speed, (24 rpm): 3.0024 MGD
     Fast speed, (48 rpm): 6.0048 MGD

2 Parshall Flumes (Lancaster and Clinton) Throat Dimensions (Clinton Only):
Width: 18"
Depth: 36" to 45"
Grit and Screenings Removal
1 Bar Rack

Type: Manually cleaned

1 3/4" clear opening

1 Mechanical Bar Screen Channel width: 30"
Channel depth: 64"
Incline angle 10O
Clear opening: 3/4"
Drive motor, (hp/rpm): 2/1800
2 Aerated Grit Chambers

Dimensions (each):
-Length: 18'
-Width: 19'
-Depth: 12.5'
-Water Depth: 14'
-Peak Capacity: 12 MGD/unit

2 Grit Screw Collectors

Type: Helical Screw

Length: 18'
Diameter: 12"
Screw Speed: 1.043rpm
Capacity: 24 (cfh)
Motor, (hp/rpm): 1/1750

Primary Treatment Process
4 Primary Clarifiers

Type: Rectangular, chain and flight
Basin Dimensions: 82'(L) x 24'(W)x 9'(D)
Overflow Rate, gpd / sqft:
-Avg. flow, 2.03 MGD: 516 (2 units operating)
-Peak hour flow, 12 MGD: 1524 (4 units operating)
-Target rate 800-1200 average, 2000 maximum

Detention Time, (hours):
-Average flow, 2.03 MGD: 3.2 (2 units operating)
-Peak hour flow, 12 MGD: 1.1 (4 units operating)
Collector Mechanisms Type: Straight line, longitudinal and cross collectors, chain and flight

4 Cross-collectors (one per basin)
8 long-collectors (two per basin)
Sludge Collector Drive Type: Helical spiral bevel gear w/overload clutches and shear pins

Drive Motor, hp/rpm: 0.5/1800

8 Scum Troughs Type: Manually operated, rotating troughs

Though width, ft/ section: 12'

Chain and flights Type: Polyester reinforced fiberglass

Long flight speed/spacing: 2 fpm /10'
Cross collector speed/spacing: 4 fpm / 5'

Trickling Filters
2 Tricking Filters (existing)

Type: High-rate roughing filter

Size, (dia x depth) ft: 60' x 5'
Media/material: 3" to 1.5" crushed stone
Depth, ft: 5'
Volume,: 14,125 cu ft (ea)

Rotary Distributors:
-Type: 4 arm, primary and secondary
-Min./max. flow, MGD (ea): 0.631/2.52

Organic Loading Rate. lb BOD5 / day /1000 cu ft:
- Average/max.: 37 / 74

2 Trickling Filters (new)

Type: High-rate roughing filter

Size, (dia., x depth) ft: 80' x 5'

Media / Material: 3" to 1.5" crushed stone Depth, ft: 5'
Volume,: 30,150 cu ft (ea)
Rotary Distributors

Type: 4 arm, primary and secondary

Min./max. flow, MGD (ea): 1.008 / 5.04

Organic Loading Rate, lb BOD5/day/1000 cu ft: Average/max.: 65 / 130
Advanced Secondary Treatment
5 Intermediate Lift Pumps

Two submersible pumps and three screw pumps

Design capacity (MGD):
- Slow speed, (24 rpm): 3.0024 MGD
- Fast speed, (48 rpm): 6.0048 MGD
6 Aeration Tanks

Basin dimensions, each: 50'(L)x 50'(W)x 17'(D)
Volume (per basin): 317,900 gals

Detention time, hrs:
- Conventional mode (2 basins): 5.0 hrs. at 2.03 MGD.
-Advanced treatment (4 basins):10 hrs. at 2.03 MGD.

3 Clariflocculators Type: Circular, Rapid sludge return

Inside diameter: 80.25'
Surface diameter: 73'
Sidewater depth: 15.1'
Volume (each): 610,200 gals
Detention time: 7.2 hrs. @ 2.03 MGD (each)
Overflow rate (each): 485 gpd / sq. ft.
Speed, rpm: 0.031 rpm

Nitrification at the Clinton Treatment Plant is accomplished in the activated sludge tanks. Nitrogen removal takes place in the activated sludge process at the aeration basins. Nitrogen removal is desirable to reduce nutrient levels to the receiving water thereby inhibiting algae growth and reducing oxygen demand on the river. Nitrification is accomplished by a biological process, which utilized nitrogen as an energy source. Proper conditions must be maintained such as dissolved oxygen supply and pH control to promote microorganism growth.

Phosphorous removal at the Clinton Treatment Plant is accomplished in a Phosphorus Reduction Facility completed in 2017. Ferric chloride and a coagulation polymer are added after the final clarifiers to precipitate phosphates into a filterable solid. Disc filters in the Phosphorus Reduction Facility capture the coagulated material for further sludge processing. Phosphorus removal at the Clinton plant limits the potential for excessive algae growth in the Nashua River due to high nutrient availability.
Dechlorination takes place at the over flow cascade of the chlorine contact chamber. Sodium bisulfite is sprayed by injectors into the effluent stream to remove chlorine residual before going to the receiving stream.
2 Hypochlorite Contact Tanks Dimensions, ft: 100'(L)x 6'(W)x 14'(D)
Total volume: 125,665 gals.
Contact time, minutes, (both tanks):
-Average (2.03 MGD): 89 mins.
-Maximum 24-hrs (8.2 mgd): 22 mins.
-Peak hour (12 MGD): 22 mins.
-Average usage: 70 gpd of 15% hypochlorite sol.
1 Sodium Bisulfite Tank

Capacity: 1200 gals. containing 38% Sodium Bisulfite Sol.

1 Bisulfite Diffuser Assembly

Dechlorination process occurs at the diffuser assembly located immediately below the discharge from the hypochlorite contact chambers.

Average Usage: 50 gpd of 38% Sodium Bisulfite Sol.

1 Parshall Flume Throat dimensions, in.:
-Width: 18"
-Depth: 36" to 45"
1 Step Cascade To increase effluent D.O. level before discharging into South Branch of Nashua River
Residuals Treatment Process
2 Gravity Thickeners

Design hydraulic loading:200 to 500 gpd/sf
Design solids loading:
- Average: 4.2 lbs/sf/ day
- Peak: 8.9 lbs/sf/day
Thickened sludge conc. (%TS) range: 3.5 to 5.5 %TS
Tank diameter, ft: 49.25'
Surface diameter, ft: 45'
Sidewater depth, ft: 12'

2 Anaerobic Digesters Type: Single-stage, Floating and Fixed Covers, gas mixed.

Tank size (Fixed Unit), ft:
- Diameter: 40'
- Max. water Depth: 28'
Tank Size (Floating Unit), ft:
- Diameter: 40'
- Max. water Depth: 28'
- Min. water Depth: 26.5'
Max. Capacity, gal (combined): 526,100 gals
1 Anaerobic Sludge Holding Tank Type: Sludge and gas holding, secondary digester, floating cover, gas mixed.

Tank size, ft: Diameter: 60'
Max. water depth: 25.33'
Min. water depth: 20.33'
Max. capacity , gal: 474,410 gals
2 Belt Filter Presses

Type: Pressure filtration, dewatering

Manufacturer/model: Ashbrook-Simon-Hartley/ Klampress, size III Type 85.
Belt width, ft: 7.22'
Effective width, ft: 6.5'
Total Dewatering area, sq.ft.: 325.34
Belt Length, ft: upper/lower: 51.5' / 53.5'
Design sludge feed rate,: gpm @%solids: 50 @ 5% to 80 @ 3%
Design polymer consumption,: lbs poly / tons sludge: 8 to 12

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River Basin Map Clinton WWTP Flow Schematic Clinton WWTP Prelminary Treatment Clinton WWTP Primary Treatment Process Clinton WWTP Trickling Filter Flow Diagram Clinton WWTP Advanced Secondary Treatment Clinton WWTP Disinfection/Dechlorination Clinton WWTP Residuals Treatment Clinton WWTP Sludge Landfill Section Clinton WWTP Sludge Landfill Plan