You may have heard of Bio-P removal, now there is a new Bio-P process of removing Phosphorus from wastewater using algae rather than the traditional chemical approach. But how do the two methods compare, and what are their pros and cons? Andrew Best of I-Phyc explains…
Ferric Sulphate dosing – background
Metal salts and most commonly Ferric Sulphate dosing can be considered to be the current industry standard method of removing phosphorus in wastewater discharges. Ferric is used to react with soluble phosphates to form solid precipitates that are then removed by solids separation processes including clarification and filtration.
To reach the discharge limits set out in the more recent Asset Management Plan periods, as low as 0.5 in AMP6 and 0.25mg/l in AMP7, often a double dose of Ferric Sulphate is required – known in the industry as two-point dosing – followed by Tertiary Solids Removal.
Ferric Sulphate dosing – pros
It is a tried and tested technology that the industry is used to. It is comfortably capable of reducing P levels down to 0.5 mg/l at larger plants, and has been shown to reach 0.25mg/l in some cases.
Ferric Sulphate dosing – cons
Ferric Sulphate has essentially been doing the P-removal job just fine for plants with higher Phosphorus consents, however consents are getting tighter and trials in AMP6 indicated that 0.25mg/l was getting towards the limit of chemical dosing technology. While larger plants have been successful in getting P levels down further, the discharge levels in the next Asset Management Period might well be not possible to reach.
For smaller plants in particular, it’s often a struggle to reach levels of 0.25mg/l with Ferric Sulphate dosing. This can be because the variable flows that smaller rural plants experience make it difficult to achieve consistency and the standard of existing assets such as settlement tanks / clarifiers and sludge holding tanks are not sufficiently sized to handle the resulting increase in solids produced by the use of ferric. As a result, there is a difficult balance of achieving the Phosphorus consent while using chemicals economically and staying within the Fe / iron consent.
There is the consideration that as a consequence of using an iron based chemical, the treatment works is also constrained by having a maximum iron discharge allowance applied, typically a limit of 3.5 to 4.0mg/l Fe and, for now, this quite high level of iron is seen as a necessary evil as a consequence of Phosphorus removal.
But that doesn’t mean to say it should be accepted forever. Added to that, achieving low P and low Fe simultaneously is actually very difficult and in practice not really viable below 0.2mg/l with Ferric dosing.
In terms of processing, Ferric Sulphate is extremely acidic so two-point dosing will affect the pH of any treatment works – areas where the water is already soft will often need additional chemicals in the form of caustic dosing to counteract this impact.
And what about the fact all these chemicals are on hundreds of wastewater treatment sites, there are health and safety considerations for operators, during the filling of tanks and maintenance of the dosing equipment, protective work-wear and emergency showers all have to be provided. Any spills of these chemicals are hazardous, not just to the operator though, but also to the environment should a leak occur.
It is also worth noting the demand for Ferric is currently increasing at an exponential rate and the supply chain will be challenged not just to satisfy the demand but also to supply at an affordable cost, chances are the cost will only go one way!
But most of all, ultimately Ferric dosing technology is nearing its limit for the amount of Phosphorus it can remove.
Bio-P removal – background
Bio-P is a relatively new technology that harnesses the natural power of algae to remove Phosphorus to levels well beyond the 0.25mg/l level often required in AMP7.
Bio-P removal – pros
It will remove Phosphorus to below 0.1mg/l and indeed, it can remove almost all phosphorus to trace levels, thus futureproofing the technology against future legislation,
The benefits of Bio-P removal using algae are numerous.
This is a chemical free process using algae to remove Phosphorus from wastewater so no transportation of chemicals, no danger to operators and no danger to the environment.
Almost total removal of ammonia; for sites with a combined Phosphorus and ammonia consent this is a double benefit, eliminating the need for SAF plants or HRF’s
No need for additional chemicals to correct the pH of the effluent.
No sludge production so existing assets are unaffected by the process and no additional transportation and disposal costs. There is a small production of excess algae (far less than the amount of waste sludge produced by ferric systems) but this is harvested off as a resource such that the phosphorus (and nitrates in the ammonia) are not lost but can be reused giving a circular approach to Phosphorus and Nitrate use.
It does everything chemical dosing does and more and it does so cleanly without any of the drawbacks or consequences we have learnt to live with.
Bio-P removal – cons
There really aren’t any cons to speak of, apart from that, the technology is relatively new and so many in the industry have not had the chance to fully get their heads around it.
Nonetheless, it is proven, available now, cost effective, and environmentally friendly. It has already been embraced by some members of the water industry and could well be one of the industry standards of the future.