The composition of the influent and the availability of easily biodegradable substrate are crucial factors for bio-P removal and denitrification. Several methods to improve substrate availability are discussed. The influence of the value of the main operational parameters on nutrient removal is indicated. Finally, a closer look is taken on how common operational problems should be resolved.
- Influence of waste water characteristics
- Improving substrate availability for biological nutrient removal processes
- Optimisation of operational conditions
- Resolving operational problems
(1) Influence of waste water characteristics
In this section it will be demonstrated that, as expected, 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. Click here to download this section.
(2) Improving substrate availability for biological nutrient removal processes
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 denitrification.
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. Click here to download this section.
(3) Optimisation of operational conditions
A nutrient removal wastewater treatment plant will have to balance the requirements set by the different biological processes involved: nitrification, denitrification and biological excess phosphorus removal. In this section it will be discussed how to attribute values to the following main parameters: (1) sludge age, (2) size of anaerobic-, anoxic- and aerobic sludge mass fractions and (3) the value of the different recirculation factors “a”, “s” and “r”. To download this section, click here.
(4) Resolving operational problems
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 realised 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 hydrolysed. 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). Click here to download this section.