Triton is key at Mountmellick

The existing wastewater treatment plant at Mountmellick dates back to the 1970s. Triton aeration and mixing units have been installed to enhance the plants energy efficiency and reduce its carbon footprint.

KEE’s Triton aeration and mixing units have been helping to reduce carbon footprint and improve plant performance following an upgrade at Mountmellick wastewater treatment plant. The existing plant serves the village of Mountmellick, which is about 15 miles north-west of Portlaoise in the Republic of Ireland.

As with all Irish towns, Mountmellick has grown greatly over the past few years. The existing plant was designed and installed in the mid1970s with further works being added later, the last being a sludge dewatering facility installed and commissioned within the past few years.

The works consisted of a Wham Mechanical Screen followed by an Oxidation Ditch for biological treatment, two final settlement tanks and sludge dewatering works. The biological treatment of the wastewater was provided by use of an activated sludge process through a race track-shaped oxidation ditch of 60m long straight lanes – each one was 7m wide – and semi-circular ends of 7m radius. The ditch’s average depth was 1.57m and the volume 1,340m?.

The main aeration equipment in use was a horizontal brush aeration system, as was the practice during the 1970s. At Mountmellick WwTP, this comprised four horizontal rotors, each 2m long. The brush rotor aeration capacity was inadequate and was supplemented by Venturi Aerators. The design capacity of the works was 5,000PE and the discharge consent was 20mg/l BOD and 30mg/l SS.

The combined horizontal brush and Venturi Aeration systems could not support the process oxygen demand and to provide the necessary velocity profile in the ditch to ensure the minimum vital mixing requirement of the ditch contents. Consequently the plant was failing consent.

In addition to the problems associated with the failure of consent, current installed power for the aeration system at Mountmellick was 42kW and was made up of two 7.5kW drives for each horizontal brush rotors and two 13.50kW drives for each Venturi Aerators.

With so much existing installed power, and the need to plan for expansion of the plant capacity from 5000PE to 7000PE, it was an opportunity to review the whole aeration system for process performance as well as power saving to reduce carbon footprint.

In 2007, Laois County Council’s Larry Gittens contacted KEE Process about its Triton aerators and mixers – he was interested in their potential capability to overcome the process issue facing the plant for the current load and explore the option to save power and increase the plant capacity.

KEE visited the site to gain an overall knowledge of the works and formulate a proposal. After further liaison and evaluation of the actual dissolved oxygen requirements, it was agreed that the plant upgrade should be carried out in two phases using the Triton dual mode fine bubble aerator/mixer.

Discharge consent

For the Phase 1 upgrade, it was necessary to bring the plant’s process performance to meet the required environmental discharge consent and reduce the overall carbon footprint. Phase 2 was to consider the plant capacity upgrade from 5000PE to 7000PE and reduce maintenance requirements.

Based on the current load and process dissolved oxygen (DO) demand for Phase 1, it was necessary to install four 3.7kW Tritons to provide the supplemental DO and all the mixing for the oxidation ditch. These four Tritons would provide complete mixing of the ditch and generate the optimum horizontal velocity of 0.3m/s, ensuring that the suspended solids in the mixed liquor remained in suspension at all times.

Although the stated environmental objectives do not require nitrification, the NH4-N in the influent would exert an oxygen demand due to high retention time of 20 hours at DWF in the oxidation ditch, the projected AOR (actual oxygen requirement) of 959kg/day accounts for both carbonaceous BOD reduction and Nitrification.

For carbonaceous BOD removal only, the AOR would be 773kg/day.

To provide the projected AOR, eight 3.7kW Tritons were required in addition to the existing horizontal brush rotors that would be retained. This assumes that nitrification would be accepted as a future treatment objective, otherwise the Tritons on their own would be sufficient to provide all the projected DO and mixing for the 7000PE capacity.

Triton aeration units are easy to install – there is no decommissioning of existing works while installation is being carried out. The Tritons’ fine bubble aerators are surface mounted and therefore the aeration tank or the oxidation ditch do not need to be decommissioned, emptied, refurbished, refilled and re-commissioned.

Tritons can be bridge-mounted, wall-mounted, float-mounted or guiderail-mounted for SBR configuration.

Timing was of the essence to ensure that the treatment objectives were met and therefore the equipment was ordered whilst the installation format was being finalised.

Mixing mode

At the Mountmellick site, the council and consulting engineering company Nicholas O’Dwyer used a float mount option where the floats were anchored to a steel bridge across the ditch with H frames attached to the bridge. This gave the option to have fluctuating water levels as the H frame pivoted the Triton floats to the bridge.

The existing DO probes were used to control the brush aerators through the variable speed drives and if the DO remained high, and the Triton blowers were similarly controlled through the DO probes.

At least four Tritons had to be operating in the mixing mode to completely mix the ditch contents and therefore for Phase 1 the control philosophy was designed only to control the Triton blowers operation to maintain the DO at between 1 and 2mg/l.

After the Phase 1 installation was completed and within 20 minutes of the four Tritons being commissioned, the mixed liquor DO concentration started to rise and it was evident that the units were re-suspending solids that had been settled out in the bottom of the ditch previously.

Within two days of commissioning, the units were achieving 1-2mg/l DO constantly, with the existing brush rotors only coming on to assist at times of heavy load when the DO concentration started to drop.

Four out of the eight Tritons for Phase 2 will be operated in aeration and mixing mode at all times and the other four Tritons would be operated in mixing and/or aeration mode. To ensure an even use, it was also decided that the Triton operation would be alternated in duty and assist mode.

For Phase 1 the installed power was reduced from 42kW to 30kW and the treatment objectives were achieved. The energy efficiency gains and optimisation objectives were also achieved by controlling the aeration mode to provide oxygen as and when required. An energy saving in excess of 28.5% was achieved and the environmental objective for effluent quality was met.

The Phase 2 upgrade is still to be implemented but it is expected that the energy efficiency gains and carbon footprint objectives will be met. The total installed power for Phase 2 will be 45kW, providing increased plant capacity of 40%.

Compared with the situation before the Triton installation, the plant performance has been regained and the capacity increased by 40% for little extra energy use. The carbon footprint can be further reduced if, as required by the consent for discharge, nitrification of the effluent was not pursued.

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