BWPCF Processes and
Equipment Descriptions

HEADWORKS:

Upgraded Brewer Wastewater Treatment Plant Schematic

The sanitary wastewater from the citizen of the City of Brewer is conveyed to the BWPCF by 12 pump station, 3 of which pump directly to the BWPCF through a force main capable of carrying approximately 13.0 MG. The sanitary wastewater will pass through the Facility's Headworks and receive preliminary treatment to remove grit, rags, paper, coarse materials, and receive flow measurement.

The BWPCF headworks has being completely upgraded during the upgrades to treat up to 13.0 MGD of wastewater. During the Phase III upgrade the Facility was equipped with a Aerated Grit Chamber with 2-grit Classifiers. For 20 years the BWPCF has been plagued by poor grit removal equipment. Grit passing through the chamber has destroyed equipment and filled lines and tanks effecting treatment. This new grit chamber and related equipment will effectively eliminate any future problems associated with grit removal. After grit removal the wastewater flows by gravity to two channels where the wastewater passes through Muffin Monster Grinders which breakdown solids in the wastewater to a particle size acceptable to all future processes. After grinding, the wastewater passes through two flumes for flow measurement and from there flows by gravity to Primary Treatment. All processed wastewater from the mill receives pretreatment and preliminary treatment separately prior to entering the Facility Primary System. All mill flow receives Primary Treatment, however the Facility operators can decide how much Sanitary wastewater receives primary treatment by adjusting valves in a distribution box.

PRIMARY TREATMENT: Sedimentation

The BWPCF Primary treatment currently consists of three 50' Circular Primary Clarifiers and two 20' Gravity Thickeners. During Phase III an additional 50' Clarifier was installed which now allows two clarifiers to be used specifically for treatment of stormwater run-off. One Clarifier is dedicated exclusively to treat mill waste.

Primary Sedimentation is designed to remove most of the heavy solids suspended in the flow, thus reducing the loading to secondary treatment. All solids which settle in the three primary Clarifier will be pumped to the two gravity thickeners where it will stay until the operators pump the solids to the Belt Filter Presses for dewatering.

SECONDARY TREATMENT: ACTIVATED SLUDGE

Selector Basin & Aeration Basins:

Effluent from the Primary Treatment flows by gravity to a Selector Basin at the head end of the two-Complete Mix Aeration Basin. All the waste streams in the facility including Return Activated Sludge (RAS) are mixed in a distribution box prior to the Selector and nutrients are added as needed. The selector is a concrete tank which allows for approximately 30 minutes of detention time, giving the hungry "bugs" returned in the RAS a chance to "select" their food prior to Aeration. After the Selector, the wastewater flows by gravity to the two .45 MG complete mixed aeration basins where a controlled environment exists which produce bacterial floc capable of settling in Secondary Clarifiers and returned as "hungry bugs" to the Selector as RAS. We call this bacterial floc "mixed liquor".

The function of the Aeration Basins is to hold activated sludge bacterial floc in contact with the wastewater long enough for absorption and organic assimilation to be completed. Four mechanical aerators mix the tanks and inject dissolved oxygen which is necessary to keep the bacteria alive and the basin aerobic. Without the aerators producing oxygen the bacteria would die, turn septic, create odors, and negatively effect treatment. All four aerators were replaced during Phase III, with more efficient, economical mechanical aerators driven by VFD's.

SECONDARY CLARIFIERS:

After at least 4 hours of detention time under aeration the mixed liquor flow by gravity from the aeration basin, to the Facilities four circular secondary Clarifiers. Two of the Clarifiers are 50' in diameter and two are 65'. During the 1992 Process study the two 50' Clarifiers were identified as being much too small to treat the current wastewater flows and loadings, therefore two 65' Clarifiers were installed during a upgrade which went on line in 1994. Since 1994 the Facility has been very fortunate in, due to the additional Clarifiers designed on " solids flux" instead of hydraulics, there has been no violation of the facility's discharge permit. This has been accomplished while sometimes treating sustained wastewater flows of twice the Facility's design limits for months at a time.

The hungry bacteria which settles to the bottom of the final Clarifier is either returned by RAS pumps to the Selector basin to help remove more contaminants in Aeration or if the Aeration Basin has to many "bugs" it can be pumped to a Dissolved Air Floatation Unit (DAF) as waste activated sludge (WAS) to receive further treatment.

The clear treated effluent flows from the Clarifier weirs to a collection launder and is conveyed by pipe to the Chlorine Contact Chamber.

CHLORINE CONTACT CHAMBER:

Effluent from the secondary clarifiers flow to the contact chamber where sodium hypochlorite is added, and adequate detention time is allowed to ensure disinfecting. This tank was reconfigured and expanded during Phase III to treat up to 15.0 MG. The original design was only capable of effectively treating 3.03 MGD and had a length to width ratio of 8:1, instead of the EPA required 40:1.

After disinfecting the treated effluent is discharged to the Penobscot River.

SELECTOR BASIN: Anoxic

The Selector basin installed during Phase II upgrade is a very unique and integral part of the overall quality operation of the BWPCF. Prior to its installation in 1994 the facility operators were continuously adjusting process control parameters and chlorinating the RAS in a attempt to control filamentous bacteria which was caused by low F:M conditions in the Complete Mix Aeration Basin. The Selector is a concrete tank installed prior to aeration which allows the hungry "bugs" returned from the secondary clarifiers to "select" there food in a environment of a much higher F:M than the complete mix aeration basin. The selector installed at the BWPCF which allows for a 30 minute detention time is anoxic which means it operates with little or no dissolved oxygen. The hungry bugs utilize nitrate as a substitute for oxygen and because the tank is operated so close to anaerobic we have demonstrated it also will remove 75-90% of phosphorous from the wastewater.

Since the Selector was put on-line in August, 1994 the facility's SVI's have not exceeded the target of 100. SVI or Sludge Volume Index is a method of process control used by operators of activated sludge plants to monitor the abundance of filamentous bacteria in the mixed liquor. Currently, the BWPCF operator can monitor and control filament growth, simply by performing daily process control and by adjusting the wasting rates according to the results, and the SVI's can be easily controlled within the target parameters. The facility is still able to produce filaments, however the growth is much more subtle, therefore allowing the operators more time to react to adjust the process control parameters.

SLUDGE DISPOSAL:

Daily, after performing process control analysis in the BWPCF laboratory on the mixed liquor,the staff must decide how much excess activated sludge exists in the aeration basins and secondary clarifiers. Excess sludge must be wasted to the DAF unit where it is thickened to 6-8% solids. The thickened sludge is stored over night in sludge storage tanks and will be dewatered the following day by blending it with thickened sludge from the gravity thickeners. The blended sludge is pumped to one of the two facility Komline-Sanderson 2.0 M Belt Filter Presses. The blended sludge is dewatered to 20-40% solids, then lime stabilized to kill bacteria before it is transported to permitted sites to be landspread. The Facility Sludge Program is approved by Maine DEP and the 5000 cubic yards produced yearly is landspread on sites throughout Central Maine.