There’s too much of it in the waterways. There’s too much (or not enough) in farm soil. What’s the future of managing this nutrient in clean-water plants?
Phosphorus is becoming a riddle for clean-water plant operators. Not so long ago, the nearly sole focus was on getting it out of effluent to keep from fertilizing rivers and lakes and causing blooms of blue-green algae.
Of course, taking phosphorus out of the wastewater generally means putting it into the biosolids. And, now, increasingly, regulators worry about applying that phosphorus to the soil, from which it can migrate into waterways if biosolids aren't carefully managed.
Hanging over these concerns is the issue of peak phosphorus: The mines that produce phosphorus for commercial fertilizers are seeing declines in volume and quality. And phosphorus of course is essential to farm crops (and for that matter to all plants and all life).
So it appears that on a global scale we have the challenge of finding ways to keep phosphorus away from where it does harm and putting it where it does good. That’s not easy, technically or economically.
Issues on land
Tight effluent phosphorus limits on treatment plants are increasingly common. Reaching a low limit often means a costly plant upgrade. Whether removed chemically or biologically, phosphorus ends up in the solids stream. More and more often, it becomes a constraint on where the biosolids can be applied.
Biosolids are typically applied in amounts to meet the crop’s need for nitrogen; phosphorus goes along for the ride. Some farm soils are naturally rich in phosphorus — there is more than enough to nourish the crops. So applying more doesn’t make much sense, especially when the excess might end up polluting a stream. Other farm soils are poor in phosphorus and need replenishing. Here, biosolids can provide a great benefit.
State regulators deal with this issue by requiring soil tests and allowing addition of phosphorus only where it’s needed. Some states use software programs that assess the pollution risk from applying phosphorus. Risk factors include cropping patterns, distance to waterways, severity of slopes, and the presence or absence of farming practices like no-till and streamside buffer zones.
For clean-water utilities, managing biosolids high in phosphorus can become a logistical problem. What if the farms with soils low in phosphorus are far removed from the treatment plant? Hauling biosolids long distances, even if well dewatered, is expensive. So is taking biosolids to landfill because there are too few close-in, permittable sites.
Perhaps this is where nutrient recovery comes is. A few processes exist that can extract phosphorus from biosolids in the form of struvite, yielding a marketable phosphorus fertilizer than can be transported at reasonable expense. Such processes have the extra benefit of helping to limit troublesome struvite deposits in digesters and related piping and equipment. Of course, phosphorus recovery takes a substantial investment; it’s not for everybody.
An alternative or adjunct to nutrient recovery is adaptive management — reducing phosphorus inputs to streams by improving farming practices throughout watersheds. This is beneficial even apart from wastewater treatment; it just so happens that wastewater utilities can become a source of funds for making the improvements. That is, instead of spending on costly plant upgrades, the utilities can help fund more environmentally sound methods of farming.
More and more, phosphorus in wastewater looks like an issue that is not local, but national and even global in scope. Phosphorus is an essential resource that is at risk of becoming scarce. It certainly makes more sense to capture it so that it can be used optimally than to spread it around willy-nilly.
I suppose I am naïve enough to think there is room somewhere, at some future date, for a national policy and guidelines on managing phosphorus from all its sources. An element among those on which life depends certainly seems to deserve that kind of priority.