Techically speaking

Black and Veatch's Frank Rogalla looks at the Ringsend works in Dublin

The Ringsend WwTW serving Dublin was tendered as a DBO contract during an open international competition. Secondary treatment with partial nitrification and sludge treatment and drying were required. Denitrification is dictated by the low alkalinity. The average design flow is 5.7m³/s.

Screening through 6mm screens and grit removal was installed for the full flow including storm flow of 23m³/s. After screening the storm flow will be retained in 300m x 130m wide basins for re-pumping to the plant. Two of the four existing primary tanks were converted to Lamella units for treating the entire full flow to treatment (FFT) of 11.3m³/s. The remaining two have been demolished to make room for secondary treatment, which will occupy only 2.4ha.

Double layer
After screening, 20 minutes aeration is provided in the aerated grease and grit removal units prior to settlement in the lamella primary tanks. Storm discharges from the flat combined sewers carry much inert suspended solids that require removal due to the long solids retention time (SRT) required for nitrification. Pre-aeration helps to achieve the required 50% total suspended solids (TSS) removal. Primary effluent is pumped to two distribution chambers, one for the upper layer of SBR units and one for the lower layer of SBR units. In each chamber flow is split to three clusters of four tanks with provision for a fourth.

The preliminary plant and lamella primary tanks have been in operation for two months. The SBR plant start-up began in June 2002.

Full Blown
Provision is made for denitrification through cell synthesis during the fill phase of each tank and endogenous denitrification during the settle and decant phases. The feed is to a partitioned anoxic zone to which mixed liquor will be recycled during the fill phase while the remainder of the tank is aerated. During the fill phase the blowers will run at maximum capacity until a dissolved oxygen (DO) concentration of 2mg/l has been reached. At the end of the aeration period, the DO will be increased to 4mg/l.
The basins were initially designed for partial nitrification but will now be operated for full nitrification and partial denitrification. The combined effluent must comply with a 95%ile limit of 5mg/l NH3-N. Control of the SRT and change in the cycle times during high flows will be described below. The cycle times for the top and bottom layer of SBR units will be staggered to ensure the UV system will never have a no-flow situation.

Primary and secondary sludge will be combined in sludge holding tanks, dewatered to 17% by belt presses then passed through a heat hydrolysis stage. Steam will be added at 12bar in a pressurised batch reactor to raise the pressure to 8bar and the temperature to 165°C for 20 minutes before discharge to a flush tank. The hydrolysed sludge will then be cooled and passed to the digesters at about 9% solids.

After normal digestion, the sludge will be centrifuged to 34% solids, then dried to 95% solids using waste gas flues from the combined heat and power (CHP) engines. The heat hydrolysis process leads to an increase in gas production and decrease in overall solids production, both important parameters in the overall life cycle cost consideration for this project. Gas produced from digestion will drive CHP engines. Exhaust gas will be used for generating steam and will provide heat for sludge drying.

Daily analysis
To manage the mean cell residence time (MCRT) for a large SBR system, it is necessary to control 24 SBR tanks individually. This may mean having 24 sludge density meters plus 24 flow meters. To waste the sludge at the highest possible concentration, i.e. not during the aeration cycle, the method selected determines the wastewater characteristics on a regular basis. It was specified that composite COD samples should be analysed daily.

Using the wastewater characteristics, the net sludge production can be determined for any temperature at the required MCRT. Thus when the daily COD composite sample is analysed, and the flow to the plant recorded, the computer system can determine the required solids inventory in the tank.
A small pump was installed in each of the tanks to pump during the aeration period to a small container fitted with two reliable sludge density meters. When a signal is given by the SCADA system, the pump in a certain basin will switch on, pumping mixed liquor to the sludge density meters. After a few minutes, the computer will read the depth of the mixed liquor in the tank while at the same time reading the sludge density in the box. This will determine the mass of solids in the aeration basin.

Wasting takes place during the last 15 minutes of decanting, from a long manifold situated under the decanters, to ensure the sludge density is as high as possible. Wasting is by gravity to sludge holding tanks. The duration of wasting will be adjusted by the computer according to the value of the solids inventory being higher or lower than the target value determined from the COD load, MCRT and temperature



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