BIOSOLIDS VALUE – SO MISUNDERSTOOD
This month’s library makes me cringe. But it is my job to showcase both sides of the story; to present different perspectives. The library is all about separating biosolids out into the different separate components, metalswith the underlying assumption that while the whole is bad, the pieces are good. This approach really hit the press this last winter with the paper that talked about biosolids as a viable source of gold, silver and other precious metals.
While that paper is included in the library, let’s start with a just accepted manuscript that was written by Peccia (out of Yale) and Westerhoff (from AZ). Both have significant publication records in well-respected journals that typically focus on the evils and hazards of biosolids. Westerhoff is also one of the authors of the mining for gold paper, so you know where he is coming from. The title of this paper ‘We should expect more from our sewage sludge’ will be offensive to many but it is also a paper that you are likely to hear about, so better to read it first.
The authors argue that our current biosolids management practices are not sustainable and do not fully recognize the valuable components present in the sludge. These include the nutrients, precious minerals, and carbon. Our current system, the authors argue, is not sustainable as it involves transport of an odorous and hazardous product greater and greater distances. Public objections to use of biosolids are forcing municipalities to spend more and more resources to produce a better biosolids. They argue that it is time for a mindset change to truly turn wastewater into a resource recovery operation rather than a waste treatment process. This mindset change requires rethinking our entire wastewater treatment system.
They go through a few examples. These in some cases showcase the flaws of the peer review process. For example, they talk about the issues with biological P removal – these being increased volume of sludge and a product with low P availability. It turns out that previous work has shown that biological P removal produces a material with P availability that is about equal to P fertilizers (see last month’s library). They do make an excellent point, that aerobic treatment releases the vast majority of the total N in the influent, wasting a valuable commodity. I am not sure if their estimate is correct on the volume lost, but do agree that this process, while it does mean you can met discharge permit requirements, is wasteful. But then they contradict themselves- saying that the volume of N and P in wastewater treatment is relatively minimal, so the big value is not in conserving them as nutrients but in keeping them out of water.
They also miss the boat a bit with their discussion of energy recovery- saying that there is no energy recovery associated with land application. Here they seem to have skipped the part about anaerobic digestion with energy recovery prior to land application.
From here onto the toxic metals and the potential value of these metals. More on that with the discussion of the next paper.
They say that the value of the components of the sludge- nutrients, metals and energy, are worth about $550 per dry ton and note that municipalities are PAYING $300-$800 per dry ton to treat and dispose of the sludge rather than recovering the assets. I would argue that the current business model is the problem, rather than the process, but that is just me. They then talk about promising technologies. One includes dispensing with aerobic treatment and going straight to anaerobic treatment. They note that this leaves N and P concentrations in the effluent very high. The discussion then moves onto combustion systems including pyrolysis. Here is seems, they have forgotten about the importance of N recovery. They talk about new methods such as hydrothermal liquefaction – turning sludge into oil with a release of the metals. What they are basically saying is that our current model is flawed and we need new technologies to recover assets in biosolids. On the one hand it is great that the paper recognizes that there are resources in wastewater. On the other, the authors present a limited and biased perspective on what is possible. Urban customers for biosolids based products being just one of those things that is not just possible, but a reality in a growing number of municipalities.
The second paper in the library is THE paper about metal recovery that has gotten so much attention. The authors sampled some biosolids from AZ as well as composite samples of archived biosolids from the last sewage sludge survey and test for a broad suite of metals and metalloids. They note that long term studies have shown that metal levels in biosolids are so high that they are hazardous. The majority of the studies that they site for this were done before 1985- before pretreatment concentrations reduced metals in biosolids by an order of magnitude or so. The one study that they site post 1985 is the paper published by McBride in the mid 1990s. The study looked at concentrations of elements in biosolids in comparison to concentrations in the Earth’s crust. Those in higher concentrations in the biosolids were those of interest to the study. The big ones were Cu, Zn, Ag (silver) and Au (gold). Copper and zinc were recognized as toxic elements, not as plant nutrients- funny how one of the authors on this paper was talking about remessaging in the first paper in the library. They put a value of the metals in biosolids at about $350 per dry ton. They don’t spend a lot of the discussion on the cost of the different processes required to extract the metals from the biosolids.
From here we can go to the 3rd paper where the authors propose treatment options to remove excess Cu and Zn from biosolids. Here is the jist- first you acidify to pH 2, then you bubble in some sodium nitrite and presto! Much of the Cu (45-64%) and Zn (70%) becomes soluble. The authors separated the liquid from the solids by high speed centrifuge. They added 20 mg L NO2-N and acid to sludge prior to digestion as the buffering capacity is lower. The authors suggest that this is a cost- effective means for metal removal from biosolids.
For the 4th paper we make the switch from metal recovery to phosphorus recovery. And you thought that struvite was the end of the conversation on that topic. The authors of this article argue that their process produces calcium phosphate, the same type of P found in phosphorus ores and so will be more readily accepted than struvite. It may be more readily accepted but the process used to make it most likely won’t be. The process centers on two types of membranes. It is a hybrid microfiltration-forward osmosis membrane bioreactor (MF-FOMBR). The forward osmosis (FO) and microfiltration (MF) membranes are operated at the same time in a bioreactor. The FO won’t let nutrients through and these are enriched in the bioreactor where they are extracted by the MF membrane. The authors fill the bioreactor with salt water to improve the process. Absolutely no discussion of the quality of the sludge from this system. Something tells me you may not be seeing a lot of this in practice.
The last paper in the library reviews starts with a similar premise as the first paper but does so in a much more agreeable fashion. The authors note the enormous successes that current systems have achieved. They go on to say that expected increases in energy costs and higher demand for nutrients suggest that current systems may not be the best out there to meet these two challenges. The authors discuss two systems that they suggest may have the potential to meet both of these objectives: the low energy mainline (LEM) and the partition-release-recover (PRR). They discuss benefits and problems with each system and seem to have a wellbalanced perspective on challenges to come.
In short, changes may be ahead but these should be understood within the context of our changing world rather than as failures of the current system. And if you have stock in a mining company- don’t sell it and by a wastewater facility in an attempt to get rich quick from the gold you expect to recover from the cake.
Sally Brown, University of Washington