ProMinent DULCOMETER PH-501 Assembly And Operating Instructions Manual page 390

screens. The combined screened sewage is pumped to the equalization tank. The equalization tank
buffers variability in the influent flow rate and concentrations of influent constituents, maintaining a
consistent flow rate and sewage strength through the membrane bioreactor (MBR) system. Sewage is
then pumped from the equalization tank to the pre-anoxic tank for denitrification.
In the pre-anoxic tank, screened sewage containing organics is combined with recycled mixed liquor
from the aeration tank containing nitrates. Bacteria use some of the organics to drive the denitrification
process, converting nitrate into nitrogen gas. This process occurs in an anoxic environment where there
is minimal oxygen. As such a pump and eductors are used to mix the tank to prevent addition of oxygen.
The denitrification process is used to meet the effluent nitrate operational target, reduce oxygen
requirements and to recover alkalinity, thus reducing chemical consumption.
Mixed liquor from the anoxic tank flows by gravity to the first aerobic tank followed by the second
aeration tank for aerobic biological degradation of the influent constituents (organics and ammonia). In
the aerobic tanks, the nitrification process converts ammonia to nitrate in order to meet the effluent
ammonia operational target. This process consumes alkalinity, so a caustic soda or soda ash dosing
pump is used to control the pH. Additionally, liquid alum is dosed into the anoxic zone to precipitate
phosphorus in order to meet the effluent phosphorus operational target. Mixed liquor flows by gravity
from the second aeration tank to the post-anoxic tank for final denitrification polishing. In the post-anoxic
tank there are minimal dissolved influent organics to drive the denitrification process. As such, an
external carbon source in the form of MicroC is dosed to supplement the organics and drive the
denitrification process.
Mixed liquor is pumped from the post-anoxic tank to the membrane tanks. The membrane tanks serve
as additional volume for aerobic biological treatment to remove any excess MicroC (which would
otherwise increase BOD in the effluent) and house the membrane filters used for solid-liquid separation.
A treated effluent is drawn through the membranes by vacuum pumps.
Since the solid-liquid separation process results in an accumulation of solids in the membrane tank, the
mixed liquor (containing both solids and filtrate) is continuously recycled to the first aeration tank. This
prevents excessive solids build-up in the membrane tank, and maintains sufficient biomass in the
anoxic and aeration tanks. The solids that accumulate in the system consist of biomass that has grown
from the influent organics and ammonia, as well as non-biodegradable solids from the influent sewage.
In order to maintain an optimal concentration of mixed liquor suspended solids (MLSS) (typically 10
g/L), a portion of the mixed liquor is periodically wasted by pumping from the Aeration Tank to the
sludge holding tank. Wasted sludge in the sludge holding tank is thickened by decanting supernatant
back to the screen tank. Thickened sludge accumulates in the sludge holding tanks until it is eventually
pumped out for disposal.
The STP general flow diagram is illustrated in Figure 4 and Piping and Instrumentation Diagram (PID)
in Figures 5 to 8. The following sections describe the STP components.
Operation & Maintenance Manual
Sewage treatment plant
February 2019
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