Optimization of Nutrient Removal
The composition of the influent and the availability of easily biodegradable substrate are crucial factors for bio-P
removal and de-nitrification. Several methods to improve substrate availability and the influence of the value
of the main operational parameters on nutrient removal are mentioned.
- 1. Influence of Wastewater Characteristics
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The proportion of easily biodegradable material in the influent influences the extent of bio-p removal. However,
it will also be demonstrated that for a given ratio of easily biodegradable material to phosphorus,
the extent of phosphorus removal will be higher at increased influent COD concentrations.
- 2. Improving Substrate Availability for Biological Nutrient Removal Processes
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Biological phosphorus and nitrogen removal are constrained by the availability of easily biodegradable material.
The limited availability of easily biodegradable COD will be further agravated when primary sedimentation
or anaerobic (methanogenic) pre-treatment are applied. The COD removal from the waste water will be higher than that
of nitrogen and phosphorus, making extensive nitrogen- and phosphorus removal more difficult.
At times there may be no choice but to resort to chemical phosphorus removal or to the addition
of an external carbon source to further the extent of de-nitrification.
However, in some cases it might be attractive to increase the concentration of easily biodegradable COD using biological processes,
as an alternative to the addition of an external carbon source or to chemical phosphorus removal.
Two main methods can be considered: (1) anaerobic pre-treatment of the influent and (2) hydrolysis of primary sludge.
In both cases the anaerobic treatment is only partial, i.e. the anaerobic process is limited
to the production of volatile fatty acids and is not allowed to proceed to the production of methane.
- 3. Optimization of Operational Conditions
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A nutrient removal wastewater treatment plant will have to balance the requirements
set by the different biological processes involved: nitrification, de-nitrification
and biological excess phosphorus removal. How to attribute values to the following main parameters:
· sludge age,
· size of anaerobic-, anoxic- and aerobic sludge mass fractions
· the value of the different recirculation factors.
- 4. Resolving Operational Problems
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Activated sludge systems designed for biological phosphorus removal are among the most sophisticated
technologies available in the field of waste water treatment. However, in practice there are several
factors that may reduce the efficiency and reliability of the treatment system.
The first factor refers to operational stability: it should be realized that the PAO in the activated sludge system are in fact
a large reservoir of phosphorus. Under adverse conditions this phosphate may be released to the liquid phase.
The second problem in the operation of bio-P removal systems is the handling of the biological excess sludge.
During anaerobic sludge digestion, a large part of the PAO biomass will be hydrolyzed.
As a consequence, the internally stored polyphosphates are also released from the cell
to the liquid phase and this can result in a very high phosphate concentration in the digester effluent (up to 200 mg P/l).