Complex upgrades needed within existing wastewater digester facility

View of the new primary digester at the Corbett Creek Water Pollution Control Plant.

By Meghal Patel and James Des Cotes

Anaerobic digestion is used at many large-scale wastewater treatment facilities in Ontario to treat and stabilize primary sludge being produced in the plant’s liquid treatment trains. This is often completed through the use of mesophilic anaerobic digestion to break down biodegradable sludge and to remove water and thicken the sludge.

The Corbett Creek Water Pollution Control Plant (WPCP) in Whitby, Ontario, is one such facility utilizing anaerobic digestion. The plant has a rated capacity of 84,350 m3/day and is owned and operated by the Regional Municipality of Durham (Durham). As sewage inflows increased, Durham realized that the plant was approaching the sludge handling capacity of its existing digester complex. R. V. Anderson Associates Limited (RVA) was then retained to carry out detailed design and construction administration for a upgrade and expansion.

DigesterDia. (m)Vol. (m³)
Primary Digester No. 2233,600
Secondary Digester No. 1202,370
Secondary Digester No. 2202,370
Table 1. Existing digester tanks and sizes.

The existing digester complex consists of one primary digester and two secondary digesters with sizes and volumes as detailed in Table 1. Also included is a digester control building that houses ancillary processes such as sludge mixing, heating, transferring and gas handling.


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At the time of the digestion complex expansion design, the plant was producing approximately 400 m3/day of co-thickened primary and waste activated sludge. Per Ministry of the Environment, Conservation and Parks (MECP) design guidelines, 15 days of hydraulic retention time is recommended for primary digestion. Based on this sludge production rate, 6,000 m3 of primary digester capacity was required, which exceeded the volume of primary digester No. 2 (PD2) by 66%. This situation could not be tolerated anymore and a project to expand the digester complex was initiated.

It was determined that sludge production at the plant’s maximum rated capacity could be as high as 710 m3/day. Using the recommended 15-day hydraulic retention time results in a total primary digester capacity of 10,650 m3. To accommodate this capacity, it was decided that secondary digester No. 2 (SD2) would be converted into a primary digester and a new primary digester No. 1 (PD1) would be constructed. The sizes and volumes of the upgraded digester complex are summarized in Table 2.

DigesterDia. (m)Vol. (m³)
Primary Digester No. 1 (New)264,680
Primary Digester No. 2233,600
Primary Digester No. 3 (Former SD2)202,370
Secondary Digester No. 1202,370
Table 2. Upgraded digester tanks and sizes.

PD1 was designed as a mesophilic anaerobic digester to match the existing PD2; therefore, it was provided with its own sludge heating and mixing systems. A hydraulic mixing system was designed for PD1 with duty/standby mixing pumps and a scum breaking system within the tank. A dedicated heat exchanger was also provided, complete with hot water and sludge recirculation system, to keep digester temperature at 35°C to maintain the mesophilic condition.

Due to limited space within the digester control building, the new mixing and heating equipment had to be installed in a new room, constructed south of the existing basement, adjacent to PD1.

To convert SD2 into a primary digester, it was provided with its own heating and mixing systems. The existing digester gas mixing system was removed and replaced with a hydraulic system similar to PD1, complete with mixing pump. Another heat exchanger was provided for this digester to maintain its temperature.

Significantly increasing primary digestion capacity resulted in two further design considerations: increased heating load and increased digester gas production. These two considerations were resolved together through the provision of two new dual fuel boilers which could utilize digester gas to heat the digesters. These boilers were installed in the main floor of the digester control building in areas previously dedicated to an old one and the obsolete digester gas mixing compressors.

To handle the increased digester gas generated in the primary digesters a new gas room was constructed on the north side of the digester control building. It was designed to fit digester gas handling equipment to remove accumulated moisture and to boost the gas pressure.

Due to the difficulty in incorporating complex piping systems into an already complicated digester facility, all design work was completed using a 3D model. This allowed the design team to check for interferences and to walk Durham through the facility to obtain feedback and comments.

No model existed for this facility so RVA undertook several techniques to prepare a base model. Existing as-built drawings in conjunction with numerous site visits to verify field conditions were used to prepare an accurate base model showing the size and orientation of existing equipment and piping. In some of the more complex areas, such as the boiler room, 3D laser scanning was employed which was then integrated into the model.

Construction for the digestion facility upgrade commenced in July 2016 and was substantially completed in April 2018. Construction and modification work in the existing digester complex required considerable coordination between the design and construction teams and plant operations as the facility had to remain in operation at all times. PD2 was in operation, receiving and transferring out sludge, throughout the entire project. For this purpose, limitations were put on the construction sequencing and temporary systems were required to be installed.

As digester gas was continuously being produced by PD2, its pipeline had to be re-routed through a temporary gas handling facility. A temporary gas hut was constructed to house gas handling equipment and to direct digester gas from PD2 to a waste biogas burner. After this temporary facility was commissioned, the general contractor was allowed to proceed with the other modifications to the gas collection system.

The next major construction limitation was on the sludge feed to the digester facility. A twinned feed line to the digester complex was installed which allowed the general contractor to take down and modify part of the existing system. During design, RVA conducted an extensive review of the pipe systems which required modification. Piping, fittings and valves were tagged and catalogued before construction so that the upgrades could be completed with minimal impact on sludge feed to PD2.

Finally, commissioning activities were completed in phases. The commissioning of PD1 was done first followed by primary digester No. 3. The boilers were started up on natural gas in order to provide heating for the primary digesters. Since the digestion process takes time to stabilize and produce gas, the commissioning of new boilers on digester gas was done later, once sufficient gas supply was available. The final stage of commissioning was reached when all the digester systems were run automatically on SCADA.

Before expansion and upgrades were completed the plant was experiencing deficient primary digestion capacity. Since the new primary digesters have come online the plant has been able to ensure an adequate hydraulic sludge retention time in the primary digesters and thereby maximize volatile solids reduction. The digesters are now producing enough gas to fuel the new boilers, which has reduced natural gas costs for the plant.

With the digestion facility upgrades completed for the Corbett Creek WPCP, the plant now meets MECP design guidelines and can handle sludge for current and future flow rates.

Meghal Patel and James Des Cotes are with R.V. Anderson Associates Limited. This article appears in ES&E Magazine’s October 2019 issue.


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