Wastewater Treatment

Wastewater Treatment

Riverhead Wastewater Treatment FacilityThe Riverhead Wastewater Treatment Facility (RHWTF) is a conventional primary treatment plant designed to treat wastewater from the City of St. John’s, Mount Pearl and Paradise. This municipal service area has a total existing population of approximately 130,000 people and a buildout population of 160,000 people. The estimated wastewater production from this region’s residential, commercial and light industrial sources is 120 million litres per day.

The Riverhead Facility has been designed to operate at up to three times the average flow during a wet weather event, when storm water is also being received and treated. The preliminary and primary treatment processes remove 30 to 40 per cent of organic material, 50 to 80 per cent of the suspended solids, and up to 99.5 per cent of all fecal coliform present in the influent wastewater stream. All of the removed material adds up to approximately 65 tonnes of solid waste that is no longer deposited in St. John’s Harbour.

What Not to Flush
Did you know many items labeled 'flushable' shouldn't be put in the toilet, and can't be processed in our primary treatment facility? Learn 'What Not To Flush' by watching this short video produced at Riverhead.

 

Wastewater Path
Wastewater is conveyed to the RHWTF through sanitary sewer and combined sewer pipelines. Wastewater from the west side of St. John’s, Mount Pearl and Paradise flows into the Shallow Pump Station. Wastewater from the east side of St. John’s, including most of the downtown area, flows into the Deep Pump Station. Each pump station includes a duty channel with a mechanical bar screen, a bypass channel with a manual bar rack, a screenings conveyance system, and a waste disposal container.

Under normal operating conditions, wastewater flows into each pump station, through the mechanical screen, and into the wet well where submersible pumps lift the screened wastewater up to the Grit Tank Inlet Channel. In the event of a mechanical failure in either pump station, the wastewater bypasses the mechanical screen and flows through the bypass channel and manual bar rack into the wet well. Each wet well is equipped with three pumps, which allows for one fully redundant backup pump in each well.

After the wastewater is lifted by the submersible pumps to the Grit Tank Inlet Channel, it flows by gravity into three aerated Grit Tanks where sand, silt, gravel and other inorganic material are settled out and other organic material is kept in suspension. Under normal operating conditions, the influent wastewater flow is divided evenly between the three Grit Tanks, but in the event of a mechanical failure, two of the three tanks have been designed and sized to handle the full wastewater flow.

The screened and de-gritted wastewater flows over weirs at the end of each Grit Tank to the Grit Tank Effluent Channel and then to the Primary Influent Channels that distribute the flow across three Primary Clarifiers. The primary clarification process removes over 50 per cent of the suspended solids from the wastewater by allowing organics to settle to the tank floor to form a sludge blanket. The clarified wastewater flows over weirs at the end of the Primary Clarifiers and into the Primary Effluent Channel.

The wastewater in the Primary Effluent Channel discharges into the Chlorine Contact Tank Influent Channel and chlorine is injected through a diffuser and mixed with the effluent. The chlorinated effluent then flows to the three-pass Chlorine Contact Tank and the serpentine configuration provides a minimum 17 minutes of contact time at peak flow for disinfection before discharge over the effluent weir. Sodium bisulphite solution is added at the effluent weir location to dechlorinate the final effluent prior to discharge to St. John’s Harbour.

Solids Path
Material removed by the screens in the Shallow and Deep Pump Stations are transported from the screens by screw conveyers, washed, compacted and deposited in a solid waste container for landfill disposal. Grit that settles to the bottom of the Grit Tanks is moved by screw conveyors to hoppers and it is then mixed with water to form a grit slurry and pumped to Grit Classifiers in the Deep Pump Station. The Grit Classifiers clean and dewater the grit and deposit it in a solid waste container for landfill disposal.

The sludge blanket formed on the floor of the Primary Clarifiers is scraped to a trough at the influent end by collectors that span the width of the tanks. A smaller set of cross-collectors then scrape the sludge into a hopper and the sludge is pumped to the Anaerobic Digesters for further treatment. The scum and floatables on the surface of the Primary Clarifiers is pushed to the effluent end of the tanks by the collectors and it can then be manually removed by tipping troughs and pumped to the Anaerobic Digesters for further treatment.

The anaerobic digesters heat and mix the solids removed in the clarifier to stabilize the sludge and reduce the volatile organic content. The volatile solids are converted to methane gas for use in the Facility’s hot water boilers. The remaining solids in the digester are pumped out and dewatered using centrifuges. The biosolids produced are then sent off-site to be composted.