New Take on Nutrients
The whole notion of recycling nutrients through land application of biosolids is poised to go from a politically correct thing to say (in certain circles anyway) to a necessity. We are starting to realize that excess use of commercial fertilizers is wreaking havoc with the environment. And if sustainability doesn’t really matter that much to you, is getting expensive to produce fertilizers and, in the case of phosphorus, we are likely running out. This makes using ‘used’ fertilizer in the form of biosolids and manures look pretty good. This crisis may do for biosolids what Macklemore did for Goodwill (https://www.youtube.com/watch?v=QK8mJJJvaes).
The first article in the library frames the discussion. It is meant as a general survey paper to draw attention to the fact that soils are important if we want to eat. It is a great summary statement and I know a bunch of the authors. But I won’t tell them if you don’t read it in its entirety. The critical point here is the discussion of phosphorus. There is a discussion of our reliance on fertilizers to boost and maintain yields. They mention that this dependence can ‘create economic inequalities or geopolitical conflicts between nations’. The author point out that both P and N are now selling for over $700 per short ton. US reserves of P will be depleted in about 20 years. Morocco has the largest remaining reserves but much of that is located in ‘disputed’ territory. The authors say that the only solution is ‘to develop a more coherent and integrated program of P (and other nutrient) recycling. Yes- that means us. The situation is similar for nitrogen. While we can fix as much N as we want (the atmosphere is chock full of it) our N use efficiency is very low. Inadequate reuse of already fixed N is a part of this problem.
So biosolids have a lot of both N and P and we have to start recognizing both the value of these nutrients and the importance of recycling them. What is the best way to do it? Typically we have looked at direct land application of the biosolids products as the best way to recycle nutrients. During the treatment process itself, the focus has been to get the nutrients out of the water with little consideration on keeping them in solid form or whether what we do to get them out of the water makes them of minimal utility to the crops. So for example, fixed nitrogen is often released as ammonia or N2 gas. Adding ammonia to the atmosphere is its own source of environmental problems. Adding nitrogen gas is not a concern per se except for the fact that you’ve just given back something that cost 4 kg of CO2 plus money to make. What are starting to show up in the literature are new methods for capturing these nutrients and also evaluations of the best ways to use them. Struvite is a critical tool here and it has been the subject of a previous library (December 08).
The library starts with nitrogen. The 2nd article is really cool. It is focused on N and provides an excellent summary of the issues surrounding N fertilizer production and waste of fixed N. It includes tables that show the relative energy costs of fixing N and of removing N from different wastes including struvite production. It also includes information on the inefficiencies of providing N to soils for use by plants. What these guys are suggesting is that the N in wastewater be recycled directly – through microbial N- into animal feed. This has a much lower energy cost and higher efficiency use rate. This is definitely Star Trek material and provides a different concept that could have large scale applications and cause us to re-imagine what wastewater plants are and do.
The third paper is more conventional, also focused on N. The authors are working on a gas permeable membrane that can be submerged in manures. The membrane works to capture ammonia. They optimized capture efficiency by increasing the pH to 9. This type of system would be effective for concentrated waste streams like animal manures. It would also be suitable for waste treatment in areas where latrine contents are brought by truck to centralized treatment facilities. It is included here to show the potential to capture rather than release ammonia.
For the 4th and 5th papers we go to P. Phosphorus will not volatilize like N and it is also only sparingly soluble. There are also chemical tools that are readily available to precipitate P out of water, notably iron or calcium addition. In other words, most of the P entering the treatment systems will stay in these systems. The common concern with wastewater P is that when biosolids are applied to meet the N need of the crop, the recommended application rate for P is exceeded. Much of the focus of work on P has been to determine what the plant availability of biosolids P is and how to reduce hazards associated with excess P. With new concerns about limited P reserves, this is shifting. The first paper provides a review of how treatment processes alter the plant availability of P. The authors compare manures and biosolids to commercial P fertilizer. Here the goal is to identify treatments to maximize P availability. They find (as others have found before) that microbial P removal in wastewater leads to higher P availability (here a good thing) as does low rates of Fe addition. Anaerobic digestion as well as higher rates of Fe addition reduce P availability (here a bad thing). The final paper is a life cycle assessment of the most efficient way to get the P in the wastewater back to the land. The authors use P fertilizer as the comparison benchmark. Direct land application of biosolids, struvite precipitation and use, and incineration with P recovery from the ash are considered. Here direct land application wins by a landslide. Not a surprise, but interesting because of the recognition of P as a resource not to be wasted rather than as an environmental contaminant.
It will be interesting to see how this recognition of ‘used’ fertility plays out in the next decade. Though this library is focused on N and P, the same case can be made for all of the other nutrients in wastewater. We are increasingly seeing concern over sulfur deficiency in soils and micronutrient deficiencies in people. Copper and zinc are also making the transition from contaminant to important resource.
Sally Brown University of Washington