How do you lower phosphorous concentrations at plant outlet?
asked 31 August 2017 asked by Testux (330 points)
Basically 2 approaches: 1) chemical by adding metal salts such as aluminum or iron to the aeration tank or 2) biological by promoting contact between re-circulated sludge and raw effluent. To delve any further, you need to provide us with more details on the context.
Thank you for your reply. Indeed, I intend to take the physicochemical route using ferric chloride, but my question is really about how one determines the quantities and concentrations to use?
For an existing wastewater treatment plant, the simplest thing to do would be to add a ferric chloride injection point (or equivalent). The size of the installation required to achieve this will depend on the current level of phosphorous, the target concentration, the volume of water to be treated, and so forth.
Thanks. So I can use a metering pump?
Yes, you'll need a metering pump. The calculation is a bit more complicated, since it depends on reagent preparation and the amount of phosphorous to be eliminated.
For physicochemical processes in the aeration tank, we usually use a molar Fe:P ratio of about of 1.5 with: Molar mass of P = 31 and molar mass for Fe = 56 and with a commercial FeCL3 preparation at 40.5% density = 1.44 containing 200 g/l Fe. To simplify the calculation: you need about 20 kg of commercial FecL3 per kg of P to precipitate.
Thank you very much for your reply. How do I determine the treatment rate? Thx
FeCl3 is the most common way to eliminate P in a WWTP, but be careful since, depending on your discharge level, you will have to monitor the clarifier (suspended solids will contain phosphorous compounds). Alum is an interesting option if you encounter problems with filamentous bacteria like microthrix. The injection point is also important (preferably where mixers are most active).
Positioning the injection point at aeration tank outlet can encourage biological work upstream thereby optimizing the consumption of ferric chloride. This is not possible with ferrous chloride, which requires an oxidation phase.
To avoid traffic and storage of dangerous iron or aluminum salts or iron chlorosulfate, you can opt for electrolysis using iron or aluminum electrodes as part of a tertiary treatment. No chemicals—only electric current. Efficiency on total phosphorus is 100% but also on all residual metals, residual BOD and COD including micro-pollutants; 20% on nitrates but zero on ammonia (just like chemical treatment). Discharged water will often be "cleaner" than the receiving environment. Like chemical precipitation, this method produces sludge, but instead of ending up with just chlorides and sulfates, you have many additional ferrous and ferric compounds that flocculate molecules that have resisted biological treatment and now form recoverable precipitates (sludge). This type of treatment is innovative since it is more efficient than the chemical route and less dangerous (continuous tension of 12-20 volts), but you must perform a thorough study prior to implementation. Its costs in terms of depreciation + operation are a little higher than with chemical treatment but the environmental benefits are worth it. There is a WWTP in France that uses this technique.
Thank you for this information.
Really very interesting—is it pretty recent? I've never heard of it... I'll look into it but if you have any additional info or sources I’d be interested!
Not exactly recent, in fact more like out of the archives. The Brits developed this technique at the end of the 19th century for the purification of wastewater, but it was abandoned with the advent of biological treatment. The technology got a second life thanks to some innovative companies seeking to treat certain kinds of industrial effluent and more recently for wastewater in small municipalities. This technique is also effective for most elements, with the exception of NH4 if you use iron as the electrode, including pharmaceutical micro-pollutants and pesticides. The advantage is the immediate starting up of a treatment facility once the electrodes are under tension, which is not the case for biological systems, which have a high inertia load. For the treatment of P alone, and not BOD5, it can tackle big flows of up to 15 m3/hour per reactor. The performance remains 100% in all cases. For P alone, you can use aluminum electrodes, but they are less effective in terms of any other parameters that may be required. In the Vosges area of France, a company has just been awarded a public contract for this type of treatment plant. The plant, however, also treats BOD. Another company in northeastern France has developed reactors for the treatment of some types of refractory industrial effluent. Tension varies between 10 and 30 volts, and the current usually depends on the type of flow, between 250 and 1000 Amperes.