Blackburn Wastewater Treatment Plant Upgrade Study

Section 4.0 - Blackbum Treatment Options
Secondary Treatment
Secondary activated sludge processes involve aeration basin(s) and clarifiers.
Wastewater enters the basins where it is aerated and mixed with a population
of preconditioned microorganisms. Soluble organic material is removed from
the wastewater by biological oxidation, and the insoluble material is removed
by physical entrapment and adsorption within the floc. This mixture of
microorganisms and organic matter is called mixed liquor suspended solids, or
activated sludge. AAer leaving the aeration basin, the mixed fiquor is
separated into a clear supernatant fraction (secondary effluent) and settled
solids (return activated sludge). The return activated sludge is pumped back to
the aeration basin and mixed with the raw influent to maintain the microbial
population. The solids retentmn time (SRT) or sludge age, is controlled by
daily wasting a fraction of either the mixed liquor or the return activated
sludge. This serves to offset excessive microbial growth in the aeration basin.
Conventional activated sludge processes are designed solely for BOD and TSS
reduction; an hydraulic retention time of only 7 h, and a sludge age of 4 to 7 d
are required for this. If ammonia toxicity in the effluent is an issue, the
retention times must be increased to foster nitrification, which is the
conversion of ammonia to nitrites and nitrates. Organisms, which perform this
reaction generally, grow at a slower rate. This modification is called extended
aeration, and involves doubling the hydraulic and solids retention times. A
small non-aerated compartment known as an anoxic selector is usually
included at the inlet to the aeration tank. This environment favours the growth
of flac-forming organisms over filamentous organisms, which enhance the
settleability of the mixed liquor, which in turn improves the quality of the
secondary effluent. In addition, it reduces some of the nitrates produced in the
nitrification reaction. Nitrates are converted to dissolved nitrogen, which
evolves as nitrogen gas to the atmosphere.
In the event that primary treatment is not included, the secondary treatment
parameters described above would require further increase by a factor of
approximately 1.5 to 2.0. A process flow diagram of the activated sludge
process as it may be applied at this site is described below. Screened
wastewater is combined with return activated sludge and enters an anoxic cell,
which in turn discharges to an oxidation ditch equipped with fine bubble
diffusers and mixers to drive the flow around the circuit. Due to the absence of
primary clarifiers, the oxidation ditch will be sized for a hydraulic retention
time of 24 h to ensure nitrification. The mixed liquor then flows to two
secondary clarifiers, each sized for 50 percent of the total flow. Emuent from