Science is Behind Biosolids Recycling

Two MABA member companies propose to apply biosolids in Goochland County, Virginia, and were faced with a crowd of 160 concerned citizens at a State Water Control Board public meeting the evening of August 19, 2015.  Their concerns seemed to crystallize around  Sewage Sludge Contents / Tip of the Iceberg, a long listing of elemental and organic constituents in biosolids that can strike even a casual on-looker as alarming.

Even those of us who have for decades been dealing with public concerns need to be reassured that science is on the side of biosolids recycling.  Robert Crockett, Virginia Biosolids Council, reached out to two scientist in our region to give perspective on the issues raised in Goochland. Over their many years of research, these scientists understand the complexity of the biosolids matrix, the science of pollutant fate and transport, the geochemical behavior of soil, the interactions of plants, soil and water, and the tools for assessing environmental and human health risks.

These two important responses to the “tip of the iceberg” pamphlet were generously provided this week. (They have been edited slightly.)
Greg Evanylo, Professor, Department of Crop and Soil Environmental Sciences, Virginia Tech.

A long list of elemental and organic compound constituents theoretically end up in sewage sludge. For all but the currently regulated pollutants (As, Cd, Cu, Hg, Mo, Ni, Pb, Se, and Zn), there are data that show that they are either present in such low concentrations or in such a chemical form as to not pose a hazard (and, therefore, not require monitoring), or, they are no longer produced and thereby not accumulating to any extent (and, therefore, also not require monitoring)..

Research studies on some of the PPCPs (pharmaceutical and personal care products) seem to indicate little cause for concern.  Some scientists continue to question the use of biosolids due to a "what if" factor for compounds whose fates have not been studied. Not every chemical compound requires testing because there are many fewer chemical functional groups under which these compounds can be grouped. Therefore, if we understand how a certain chemical group imparts toxicity, carcinogenicity, or endocrine disruption, we can infer how the larger numbers of these products having the same chemistry would react. Scientists can then address the concerns exhibited by compounds posing the greatest risk due to a combination of concentration, bioactivity, and presence in the environment from biosolids use.

The greatest unknown today is the potential for synergistic, additive, or multiplicative effects. These have not been demonstrated, but are routinely conjectured as concerns. For perspective, we need to remember that pathways for many of these products from source (e.g., flame retardants in carpets, computers, chair cushions) to human is much more direct from the products that incorporate these chemicals than from land where biosolids are applied.

As more wastewater facilities move to Class A or better treated Class B products, the minimal risk of pathogens previously observed has been reduced even more so.  The greatest evidence for the lack of risk from land application is the lack of evidence of people getting sick (editor note, or biosolids workers) where the biosolids have been used for many years.

Rufus Chaney, Senior Research Agronomist, USDA-ARS-CSGCL, Beltsville, MD

Handouts like “tip of the iceberg” raise concerns in those people who don't know the science of biosolids, or even general agricultural/environmental science.

Everything on Earth contains trace levels of all 92 natural elements. Soils contain certain levels mostly because the elements are either insoluble over millennia, or added in small amounts in fertilizers to maintain soil fertility. The USGS 2013 reports summarizes the analysis of thousands of soil samples collected across the USA.  I find this provides some perspective if one is seeking science rather than fear.

Soil elements are mostly so insoluble that they are not taken into plants even at the low parts per billion level. We actually measure some elements in plant samples to estimate the soil contamination of crop samples from windblown dust, or soil contact during harvest -- elements such as titanium, zirconium, yttrium, chromium and most of the rest of the rare earth elements. Even with soils geologically rich in these elements, plants do not accumulate the element, but dust contamination can be estimated by analysis of several of the elements compared to levels in the fine particles of soil.

In testing uptake by plants, some scientists add fresh spikes of soluble metal salts. But research has clearly shown results from such testing are severely flawed and have no bearing on uptake of elements equilibrated and weathered in soils, or those applied in stabilized biosolids. Huge errors have been reported in research purported to represent biosolids but instead have used spiked element additions. Some elements do reach edible plant tissues grown on biosolids amended soils, and those that are common in biosolids were subjected to risk assessment and development of the 503 regulations. There is simply no information which shows that, after long term use of biosolids as a fertilizer, foods grown on the amended soil would comprise risk to highly exposed individuals based on the home garden model (which gives much more potential for exposure than the general agricultural market model).

The same principles have been found to apply to organics and xenobiotics. Yes, many chemicals used by society can be found in biosolids, at least at measureable levels now that we have extremely sensitive methods to measure these compounds. In most situations, the use of consumer products in our homes provides a massively higher exposure to humans than could be possible from biosolids used in agriculture or even in garden fertilizer/soil conditioner products. Soluble compounds move more to the effluent, and lipophilic compounds that persist thru aerobic and anaerobic treatment at the POTW can reach the biosolids.

One of the important lessons of risk from lipophilic compounds came from study of PCBs. In the 1970s, science discovered PCB contamination from common commercial use of PCBs without attention to industrial pretreatment, thereby resulting in some biosolids quite high in PCBs. Study of pure chemical PCBs spiked to soils showed some uptake to the peel of carrots, and lesser-chlorinated PCBs even volatilized from soil to shoots of plants. But when biosolids-borne PCBs were examined, even transfer to carrot peels was significantly lower than occurred from spiked soils. Eating soil by livestock was the only way for detectable levels of biosolids-applied PCBs to reach livestock tissues.

Risk assessment of other biosolids-borne xenobiotic thus needs to test potential uptake from soils amended with appropriate levels of the xenobiotic in biosolids. Tests preferably should not have 1000-times higher concentrations than is commonly found in US biosolids, as has been the case in some "scare" experiments. Based on this view of appropriate experimental tests, no xenobiotics listed as health concerns comprise risk to humans. There is likely not even a risk to soil organisms, because the compounds are so sufficiently insoluble that they are bound strongly by the organic matter in soil and when added with biosolids.

Regarding the pathogens, EPA and most state regulations require management which prevents transfer of pathogens in Class B products to humans or the environment at levels of concern, and the increasing production of Class A products is further protection of all of the environment. Class A is better in the sense that it requires less management supervision to prevent risks, and POTWs are increasingly moving toward Class A products to win public acceptance. But that is not needed to meet the low level of risk required by US-EPA and states, which is achieved by the Class B use management requirements.

Scientists who have been involved in biosolids constituent risk assessment and development of regulations recognize the flaws in experiment design that have allowed some scientists to publish research that is purported to show a risk from nanoparticles, xenobiotics or trace elements in biosolids. Bad science is simply bad science. Only proper experiment design can test whether a constituent of biosolids can be transferred from the biosolids to plants or animals. Rates of application relevant to biosolids, in the matrix of biosolids products ready for land application, have to be the standard for testing these potential risks. Occurrence is not evidence of transfer to foods. And study of spiked soils or nutrient solutions provides no evidence relevant to real world risks from constituents of biosolids.