Ongoing plant developments
Jersey has steadily improved its effluent treatment since first commissioning a WwTW in 1959. Boyd Bennie and Duncan Berry discuss short and long-term objectives for the plantMost of Jersey’s waste disposal operations are located in Bellozanne Valley, 2km to the west of the island’s capital, St. Helier. The Bellozanne site is unique in that it generates electricity from both liquid and solid waste and provides about 5% of Jersey’s electricity. An executive arm of the government, the Public Services Department (PSD) is responsible for all liquid waste collection and treatment. Through taxation central government provides the PSD with funds for capital works and ongoing operational costs.
Up until the introduction of the Water Pollution (Jersey) Law, 2000, the sewage undertaker voluntarily complied with the Royal Commission standard of effluent, ie 30mg/l suspended solids and 20mg/l BOD. Now the regulator, also within the PSD department, issues discharge consents which can be enforced.
The Bellozanne plant deals with an 87,000PE, which increases to 100,000 in the summer. It has been designed to accommodate growth over the next 15-20 years. Just under 40,000 properties on the island are connected to the mains sewer. The remaining 13% relies on private treatment plants or septic tank/soakaway systems. There is very little trade waste with the exception of dairies, breweries and launderettes.
The WwTW is fairly conventional. As crude sewage passes through the inlet works, grease and grit are separated into a dedicated underground digester. Four primary tanks then remove approximately 50% settleable solids and 30% of the BOD. Following primary treatment, waste previously entered a plug-flow activated sludge process, but this is currently being modified. Treated effluent then goes to the UV plant, after which it is discharged into St. Aubin’s Bay on the south coast of the island.
There are a number of wastewater projects underway, which will ultimately contribute to a total upgrade of the entire plant. “We are currently upgrading our secondary treatment to increase capacity,” says Boyd Bennie, director of waste. “The inflow to the old works was 1,000 l/s peak flow and only 600 l/s could go forward for secondary treatment.
“The remainder got primary treatment, overflowed into a storm weir and went through the UV plant. During storm conditions a certain amount of effluent was not reaching the required standards,” he explains.
“We are now increasing capacity of secondary treatment up to 900 l/s and we are going to increase the flow to the works itself from 1,000 l/s to 1,300 l/s at a later stage of the work.”
A denitrification system has also been included in an attempt to reduce nitrogen in St. Aubin’s Bay. A survey of nitrates and phosphates within the bay revealed that algal blooms and seaweed growth was nitrogen not phosphorous driven.
“As part of upgrading the system we looked at nitrogen removal,” says Duncan Berry, manager of liquid waste.
“Prior to that it was calculated that 50% of all nitrates going into the bay were from the WwTW’s effluent. The island also tips from high in the north to low in the south, so all the valleys discharged into St. Aubin’s Bay,” adds Boyd Bennie.
With denitrification at the WwTW, the PSD hopes nitrate discharge to the bay will be reduced to just 10%. In the past two years the desalination plant has also been used to produce nitrate-free water for blending down nitrates. “Nitrate removal plants were considered, but we would have had to look at a method of disposing of concentrated nitrates,” explains Howard Snowden, manager of the Jersey New Waterworks Company. “The desalination plant is not as controllable, but can be used to reduce nitrate levels.” Other upgrade work at the WwTW is due to ageing equipment. The existing UV plant is now ten years old and comprises some of the oldest UV technology. “Maintenance and running costs are extremely high. New plants offer lower running costs, less bulbs, higher output and automatic cleaning,” says Duncan Berry. “With a new plant we will also get on-line dose monitoring, which is a requirement of our discharge certification under Water Resources.”
The contract to replace the UV plant has recently gone out to tender and a full working system is expected by May 2003.
New sludge digesters were installed at the WwTW during the mid-1980s and are also in need of extensive refurbishment, possibly replacement. A strategy review in terms of sludge handling and disposal is currently underway.
At present, sludge removed from the settlement tanks passes to the digestion plant where it undergoes anaerobic digestion. Through this process more than 3,500m3 of biogas is produced each day, which is then used to generate electricity and heat the anaerobic digesters.
Digested sludge is treated using a Swiss Combi advanced automated drum drying plant, also fuelled by biogas. Approximately two-thirds of sludge is dried into pellets, while a third is applied to land.
Leeds-based consultants Carlbro began the sludge handling study during the early part of 2002 and produced a short to medium-term strategy by the summer. The initial stages involve modifications to the sludge holding tanks and improvements in sludge thickening. This work is due for completion in spring 2003.
A long-term sludge strategy review is also expected to be concluded in the early part of 2003. This will consider the ultimate disposal routes for sludge and may involve upgrading the sludge dryer.
In addition to this work, the PSD has an ongoing programme of refurbishing and expanding the existing sewerage network, which consists of more than 350km of sewers and 105 pumping stations. The sewerage system is predominantly in the southern part of the island, where the population is concentrated.
The infrastructure is in need of repair, particularly in areas like St. Helier which has Victorian brick sewers. Significant relining and replacement work has already been undertaken.
Pumping stations have also received upgrades with additional stormwater storage capacity. The largest is an underground cavern in the rock beneath Fort Regent, a leisure facility in St. Helier.
“Although the pumping stations are meant for the foul sewage they do get quite a lot of surface water. In the past people have made unauthorised connections and a lot of the older parts of the system were built as combined,” explains Bennie. “We have a policy of separating as part of the reconstruction programme, but there is a limit as to how much we can do. Its obviously going to take time to achieve.
“We are also extending the foul sewer systems to quite a number of areas,” he stresses. “We have plans of all the areas that need sewers and ultimately, if funding continues, we hope to extend to them.”
Funding is becoming more difficult because the island’s budgetary situation has changed and the capital programme has been significantly reduced. In previous years the PSD received £5-6Mpa to reconstruct and extend the sewers. This has now tapered to £3-4Mpa. “We were also planning to upgrade the primary treatment at a cost of around £4-5M, but that has been put on hold and we are upgrading the UV plant instead,” claims Bennie. “We are also doing some optimisation works throughout the whole plant to improve automation and efficiency. We are going to be spending about £3M on those aspects and a further £.5M on sludge handling.
“Beyond that we were looking at upgrading the sludge dryer, probably duplicating it, at about £5-6M. We also want to replace or renovate the sludge digesters in the next 4-5 years.
“We are considering ‘user pays’ and the introduction of a sewage charge in a year or two. I am not saying these projects won’t happen, but at the moment the funding is uncertain.” Bennie concludes