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
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.
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.
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
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