State-of-the-art – and all set for the future

A new wastewater treatment plant at the heart of NI Water's Antrim Wastewater Project was opened last year. Veolia Water Solutions & Technologies' process commissioning engineer, Nicolas Tardy, describes the new works

Milltown Wastewater Treatment Works (WwTW) was built in the early 1970s and was designed to serve a population equivalent of 30,000, with final effluent being discharged to the environmentally sensitive Lough Neagh. Thirtyfive years on, an increasing population and the implementation of the Urban Wastewater Treatment Directive meant that the works was no longer suitable for the current demands.

As a result NI Water commenced a £23M upgrading programme for the area, including improvements to the existing sewerage system, with treatment being centralised at a new wastewater treatment works and sludge processing centre on the Milltown Road site. Based on a 2035 design horizon population equivalent of 80,600, this would have to treat a dry weather flow of 14.5Ml/day.

The contract for design and construction of the new works was awarded in 2007 to BSG Civil Engineering, in conjunction with Williams Industrial Services and Veolia Water Solutions & Technologies (VWS). VWS was appointed to carry out the hydraulic design of the works, the process design of its biological treatment plant, the supply/installation of strategic process equipment and odour control technology, and to oversee process commissioning.

The new treatment plant is based on a conventional activated sludge process but with biological nitrogen removal, by nitrification and denitrification, and chemical phosphorus removal. Two low level gravity sewers (1,200 and 600mm) discharge into the existing inlet reception chamber from which three new screw pumps, rated at 500l/s each, lift the effluent to 4mm bar screens, followed by a detritor to remove grit and heavy solids.

The flow to primary treatment is limited to twice the dry weather flow with higher flows diverted to a storm management system comprising one blind and four overflow storm tanks with a total storage capacity of 3900m3, giving a minimum retention time in excess of one hour at diverted Formula A flow conditions. The storm system utilises existing process tanks from the original Antrim works; the blind storm tank being one of the original primary tanks fitted with a storm tank ejector cleaner pump, and the original final settlement tanks which were converted into overflowing storm tanks with storm tank ejector cleaner pump for cleaning during emptying. The influent pH is monitored and an emergency dump to storm is automatically initiated if it falls outside the acceptable range of 5.5-8.5.

Primary treatment uses two of the existing primary settlement tanks, which have been fully refurbished with new half bridge scrapers and scum removal. They operate at a surface overflow rate of 1.4m/h and produce about 100m3/day of primary sludge at 2-3% dry solids (DS), which is removed by automated telescopic desludging bellmouths and then pumped to the new sludge treatment plant. Primary settlement removes 28-36% of the BOD and 55-65% of the suspended solids.

Secondary treatment is by a biological nutrient removal (BNR) activated sludge plant. It consists of a rapid mixing chamber where a top entry mixer ensures good mixing between the raw wastewater, returned mixed liquors and returned activated sludge.

There is also a caustic soda dosing facility to add alkalinity, if required, under automatic control by a pH probe in the mixing chamber. The mixed streams are then divided between two parallel 4,000m3 anoxic zones fitted with submersible mixers, where denitrification takes place, each supplying three 2,100m3 aeration lanes. Loads entering the plant are lower than the design, and only four of the six aeration lanes are currently being used.

Air is supplied by three blowers with distribution provided to all 12 aeration zones through actuated valves controlled by dissolved oxygen probes. Ferric sulphate is dosed at about 10mg/L Fe into each operating lane to co-precipitate phosphorus as ferric phosphate and ensure a final effluent total phosphorus concentration below 1mg/L. There are four new radial flow final settling tanks with half bridge scrapers and a set of parabolic configuration scraper blades. Each has a volume of 1900m3 and operates at a surface overflow rate of 0.87m/h. At 3.8m the sidewall depth is greater than a more conventional design, made possible by the Veolia Water process design, but this enabled a reduction in the clarifier diameter. Sludge depth is monitored by a blanket monitor, which controls actuated telescopic desludging bellmouths, and supernatant turbidity is also monitored.

Return activated sludge is pumped to the rapid mixing tank of the BNR activated sludge plant to ensure that mixed liquor suspended solids and sludge age are adequate to achieve heterotrophic denitrification and carbonaceous degradation. The return activated sludge flow is controlled by flowmeters and variable speed pumps to be equal to the influent raw sewage flow up to twice the dry weather flow.

The surplus activated sludge pumping facility removes excess sludge to control the mixed liquor suspended solids concentration in the aeration lanes at the appropriate level to ensure that there is sufficient, but not excessive, biomass. The variable speed pumps are controlled by suspended solids probes in each aeration lane and the plant currently operates with mixed liquor suspended solids between 1,800 and 2,500mg/l and a sludge age between 13 and 2 0 days, depending on temperature. Final settled effluent passes through a effluent chamber where turbidity and ammonia are measured with online probes before being discharged to Lough Neagh.

The new sludge treatment plant thickens and dewaters not only site-generated sludge, but also imported sludges from other treatment facilities and septic tanks from the surrounding rural area. Thin imported sludges are screened before being mixed with the indigenous surplus activated sludge. The mixed sludge is then thickened before being pumped to sludge tanks. Here the primary sludge from site and imported thickened sludges are added before the mixed thickened sludge is centrifuged to a minimum of 27% DS.

The resulting cake is then stored within skips before being transported off site to an incinerator in Belfast. The centrate and liquors resulting from the thickening operations are pumped back to the head of the works for integration with the raw wastewater.

In order to comply with the strict odour standards, two odour treatment systems are provided, both using Veolia’s Alizair technology. The process uses autotrophic bacteria growing on a maintenance free, expanded mineral media called Biodagene to oxidise sulphides, mercaptans, ammonia and amines, giving up to 99% reduction in odorous species.

A polishing filter charged with copper-impregnated carbon removes any residual mercaptans in order to eliminate any risk of nuisance. The main plant, consisting of three ALC4 packaged units, treats a total air flow of 20,000Nm3/h from the sludge treatment plant.

A second plant extracts and treats the polluted air from the inlet works screw pumps, leachate tanks and screenings room. The odour control systems more than meet the target of < 1000 OUE/m3 at their respective discharge stacks in order to achieve the requirement of 5OUE/m3 at the nearest sensitive receptor. Northern Ireland Water’s project manager, Kieran Grant, says: ”The construction of the state-of-the-art wastewater treatment works at Milltown Road, coupled with the refurbishments of pumping stations, the laying of new sewers and the closure of troublesome CSOs in the network, is having a hugely positive impact in the quality of the water in Lough Neagh.”

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