Bill Nye, The Science Guy, that great popularizer of science (one of my favorite media personalities, perhaps behind Jon Stewart, Stephen Colbert, and Mr. Rogers), was asked: “What Science Fact Blows Your Mind?” His reply, “We are made of dust from exploded stars.” That is, the elements that make up the human beings are elements that formed during a supernova 4.5 billion years ago, from which our Sun and all planets in the solar system aggregated and congealed. Having listened to the Great Courses lecture series on Earth’s geological history, I can tell you another fact. We may be stardust today, but in the distant future -- the very, very distant future -- we are IRON. That is, all of the transformations of elements through star forming and star dying processes in the Universe lead ultimately to that most stable of all elements, IRON.
Yes, I know, we humans have more to worry about than that the stuff of human beings one day becomes iron. But if you are a wastewater or biosolids manager, you have today much stuff to contemplate with iron.
Iron came up during last week’s MABA Annual Symposium, “BIOSOLIDS: the Sustainable and Recoverable Resource.” One question left hanging in the air during a panel session was “does the use of iron salts in wastewater treatment systems interfere with recovery of resources, particularly phosphorus?”
I personally have spent a lot of time while at the Philadelphia Water Department contemplating iron. Our water treatment operations used an industrial waste acid, “pickle liquor,” a ferric solution left over from duPont’s manufacture of paint, as a principal ingredient in production of drinking water. The residuals from its use were flushed to the sewer, and the total loadings of metals in our biosolids were notably higher than normal biosolids as a consequence. I always saw “irony” in industrial waste being used for drinking water treatment...
Over the years I sought evidence of effects of water treatment residuals discharge into sanitary sewers. On the negative side, iron increased the total quantity of biosolids; the iron was agronomically excessive; iron may have contributed to excessive self-heating in our biosolids compost; the heavy sludge settled out in our poorly mixed digesters; and iron was robbing the sewage treatment process of soluble phosphorus needed for secondary treatment. But against this were the economies -- good primary settling, low odors at the wastewater plants, only minor struvite build-up, and improved biosolids dewatering. Now that Philadelphia’s biosolids ends up as heat-dried pellets, the downside list could include low BTU per pound, brittleness, low percentage of organic matter and nutrient content in the pellets and include wear and tear on centrifuge and dryer parts.
But the issue that was standing out for the folks at the MABA symposium last week was the question whether iron discharge to the wastewater plant fundamentally interfered with phosphorus capture as a promising aspect of resource recovery. With a dozen technology companies out there with a goal of taking P out of wastewater, should wastewater operations continue their heavy reliance on iron?
An early finding by odor researchers in the late 1990s was that iron addition can positively reduce the odors in centrifuge-dewatered cake. A WEFTEC presentation in 2002 of research at PWD, Effect of Chemical addition on Production of Volatile Sulfur Compounds and Odor from Anaerobically Digested Biosolids, concluded that the “addition of FeCl3 (0, 24, 63, 125 g FeCl3/kg DS) resulted in a decrease in the VSC [volatile solids compound] production as well as odors measured by the odor panel. The decrease in odors was proportional with the FeCl3 dose and thought to be associated with the binding of sulfur compounds by the iron.”
Recent research suggests that iron could have some unexpected benefits for the dewatering of biosolids. A 2014 paper described a special iron-based bioreactor that aided dewatering: Enhanced dewaterability of anaerobically digested sewage sludge using Acidithiobacillus ferrooxidans culture as sludge conditioner. Its authors report that “results clearly indicated that the culture and filtrate of the A. ferrooxidans facilitated rapid sludge dewaterability while the cells supplemented with Fe2+ also enhanced dewaterability but required 2–4 days.” A second research team worked with MION, or magnetic iron oxide nanoparticles, and found it to be “efficient in rapid separation of sludge with very low water content, and thus could be a suitable alternative for sludge sedimentation and dewatering in wastewater treatment processes” (Effective water content reduction in sewage wastewater sludge using magnetic nanoparticles).
The sewage discharge of iron-laden water treatment residuals benefits the sewage collection system. In Feasibility of sulfide control in sewers by reuse of iron rich drinking water treatment sludge the authors explain that in “this study, we experimentally investigate the feasibility of using iron rich drinking water treatment sludge for sulfide control in sewers.”
Iron can improve important aspects of biosolids for land application. The WERF research into P solubility in the early 2000s, of which PWD was a sponsor, determined that high iron in biosolids reduces the potential for biosolids-borne phosphorus to be released to groundwater or to surface runoff. WERF sponsored research (WERF 99-PUM-2T) The Agronomic and Environmental Availability of Biosolids-P (PHASE II), showed that “TDP [total dissolved P] levels for the first runoff event were consistently lower when the total molar Al + Fe content of the P sources was high. The source of high iron could be ferric additions to treatment processes to control odors and P in effluent discharges, but it could also be water treatment residuals (WTRs). Another science article by O’Connor, Influence of Water Treatment Residuals on Phosphorus Solubility and Leaching, reported that “for the Largo biosolids treatments, all WTRs retarded downward P flux such that leachate P was not statistically different than for control (soil only) columns.” These findings have led to some states in the MABA region deploying the 'Water Extractable P', or WEP, test for farmland applications of residuals.
We learned in the study of high-iron biosolids that P bound to iron in biosolids did not raise a threat of P nutrient deficiency. Dr. Rufus Chaney vigorously put forth the point of view that P bound to iron is not unavailable to plant roots, and what is more the iron helps prevent loss of P from the root zone through groundwater or surface waters. His presentation is available on the MABA website, Influence of Phosphorus Forms in Residuals on Plant Uptake and Potential for Environmental Releases
What is more, high-iron biosolids may have distinct benefits of reducing pathways of exposure to other pollutants. The work by Dr. Sally Brown showed that high-iron biosolids used as a feedstock to compost could result in a soil amendment capable of beneficially reducing risks in garden use of contaminated urban soils. Dr. Brown wrote in High-Iron Biosolids Compost–Induced Changes in Lead and Arsenic Speciation and Bioaccessibility in Co-contaminated Soils “the results of this study indicate that addition of high-Fe biosolids compost is an effective means to reduce Pb [lead] accessibility only for certain types of Fe-rich materials,” in this case WTR-containing biosolids.
What is more, while traditional chemistry indicates that, once bound to iron, P cannot be extracted for re-use except with strong acid, this may not be true. Some exciting new research has successfully demonstrated innovative approaches to capturing back the P from high-iron biosolids. In the report Scale-up of phosphate remobilization from sewage sludge in a microbial fuel cell “phosphate remobilization from digested sewage sludge containing iron phosphate was scaled-up in a microbial fuel cell (MFC).” In another research project, P was extracted from biochar of a pyrolyzed high-iron biosolids. The paper Phosphorus recovery from sewage sludge char ash concluded: “The combination of sewage sludge pyrolysis, combustion or gasification of the char and phosphorus extraction from the final solid residue contributes to the integral exploitation of sewage sludge.
Also, a high-iron biosolids ash may have a wholly new beneficial use. In the recent article, Beneficial use of aluminium and iron components of sludge incineration residues in ceramic materials, the authors report the happy news that “With various colours and microstructures formed, the products can be used as pigments, in construction, and as ceramic membranes. The findings of this study suggest a promising process for the beneficial use of waste aluminium and iron as ceramic raw materials, and may provide an economical means of reducing environmental concern over solid wastes.”
Well, all said and done, having spent so many years addressing the "problem" of heavy metals in biosolids, I now find that the heaviest of heavy metals, iron, can have a lot of positive attributes, which to me is the irony of biosolids iron.