How is an optimal dosage determined when using alum for phosphorus removal for the final effluent in a wastewater MBR?
Alum dosage must be tailored to meet your specific wastewater characteristics and discharge requirements. It all depends on the wastewater characteristics, the mixing & dispersion conditions, the phosphorus concentration in your raw water and the expected concentration at the release point & so on…
Theoretically, one mole of Al is required to react with one mole of P to precipitate in the form of AlPO4. In practice, the aluminum to phosphorus molar ratio to be applied is greater than the stoichiometric ratio. Part of the aluminum cations Al3 + precipitate in parallel with hydroxide ions OH- to form precipitates Aluminum hydroxide Al (OH)3. This precipitation simultaneously increases the consumption of aluminum, but these hydroxides also contribute to the flocculation and decanting of the phosphor particles.
A high level of reduction requires a high Al: P molar ratio. Furthermore, a small concentration of P, which is the case of diluted municipal waters, also requires a strong molar ratio to achieve a low resulting concentration, despite a small percentage of reduction.
2. Is there a general amount of P removed by biological/chemical treatment?
* With no addition of chemical products, a primary decantation usually removes between 5-10% of the total phosphorus.
* An activated sludge retains between 10 to 20%.
* Alternating between anaerobic and aerobic zones (Biological phosphate removal) can remove more than 20% of the total phosphorus without the use of chemical.
* A chemical treatment will transform soluble phosphorus into particulate phosphorus, allowing the reduction of total phosphorus following a "Solid-liquid" separation. Alum and ferric chloride are the most commonly used coagulants in MBR process because they can reduce phosphorus to the lowest concentration.
3. Will type of coagulant make difference for P removal?
Various inorganic coagulants can be used to remove phosphorus in biological wastewater treatment. Depending on the type of coagulant used, the dosage will vary. It’s important to refer to the technical data sheets of the product. The data sheets will mention the density as well as the percentage of alum (or other) in each product. Knowledge of the product used allows a refinement of coagulant dosage calculations. Iron requirement is greater than the aluminum requirement in terms of chemical dosage to remove same amount of phosphorus.
4. How will shock loads of infiltration water affect P removal?
Infiltration will change the characteristics of our influent. It will delude the concentration of P in the wastewater. You will need to apply a strong molar ratio to achieve the required results. Also, keep in mind that a higher flow can reduce the detention time and inhibit flocculation and settling.
5. Do deficient conditions in: anoxic/aeration/MBR tank zones have an impact on P removal?
Anoxic zones are used along with anaerobic zones as part of biological phosphorus removal. Deficient conditions in your treatment process will inevitably have a repercussion on not only your P removal but on all your parameters.
6. How do you know when over/under dosing for P removal?
You control your process thru monitoring your process. To monitor the amount of phosphorus, and the required dosage of coagulant, you will need to sample various locations to take suitable actions. The phosphorus discharged from your treatment plant need to meet your discharge requirements.