ERI Home IRR Home Last updated 8/11/04
Future generations will read accounts of our standard wastewater disposal and treatment practices with shocked disbelief. "Wait a minute -- you're telling us that households were flushing their wastes down pipes with huge quantities of drinking water? That industries were using those same pipes to get rid of their industrial process wastes? That central plants were mixing it all together in a big stew, trying to treat it all at once, and then discharging the liquid fraction right upstream of the next community's water supply? And taking the solid fraction, that now inseparable blend of good fertilizer and toxic industrial wastes, and spreading it over farmland? You've got to be kidding!"
Nope, kids, we're not. That's the way it was. And that's the way it will be for the foreseeable future, given the enormous capital costs involved in retrofitting the system to create anything resembling a rational and responsible way to handle the odd collection of waste streams now running through publicly owned treatment works (POTWs).
In fairness, the 16,000 POTWs that now serve 173 million people in the U. S. (60% of the nation's population) did not design the system, and do not make the rules. Furthermore, for all the built-in inefficiencies and absurdities that the present system exhibits when observed in isolation from its historical context, it represents a giant step forward from the situation several decades ago, when the release of waterborne wastes into U. S. waterways was essentially unregulated (and the results were all too obvious).
This document will present an overview of how, and how well, the present system serves its intended function of dealing with waterborne wastes while minimizing the associated environmental impacts. There are, however, two aspects of the story which deserve to be told, but which are outside the scope of this series. The first is rooted in history. How much worse off would we be if POTWs were not performing as they are at present, and our historical methods of dealing with waste were being applied to today's waste streams? The second concerns a road not taken, or at least not yet taken. What alternatives are there to the present system that might prove far preferable in the long run?
For what we have done in creating the present infrastructure is to take what could have been an area of individual responsibility -- holding households and industrial facilities responsible for dealing with their own wastes acceptably and responsibly -- and socializing it. We have turned sewage management into a collective responsibility.
Of course, many households are, even in this era of publicly managed health, held individually responsible for sewage waste management. A well-established regulatory apparatus already exists for ensuring the safety of rural and exurban septic systems, generally administered through county health departments. However, few public health officials would for a moment consider extending the present system to overseeing the proper operation of dry composting toilets for every urban and suburban household, condominium, and apartment complex in their jurisdictions. Far better to build the pipes, and regulate a single outfall. If the urban public is deemed inherently incapable of managing its own waste, this would have to be considered the only practicable course.
Responsibility for dealing with wastes has indeed not been as thoroughly woven into our cultural assumptions as other types of responsibility. But that might be changing. The National Pretreatment Program is a step in that direction. Industries at least are expected to take the initial steps to mitigate the impact of their wastes on those who must deal with the consequences downstream, including both the operations POTW that first receives it, and ultimately whatever is touched by the effluents and wastes from the POTW.
The success of the National Pretreatment Program will strongly influence the outcome of an issue with the potential for significant impact on the future operations of POTWs -- the appropriate disposition of the solid waste fraction from secondary treatment. An increasing proportion of this waste is being used as fertilizer on food crops. The risks associated with this course are outlined below. This may become a paradigm of appropriate waste management, or it may be the environmental sleeper issue of the coming decades. Willingly or not, the POTW is the institution that will play the pivotal role in this unfolding situation.
While it has not been without serious problems of design and implementation, the National Pretreatment Program is at least a well-established starting point. Who knows -- someday, even the general public may prove to be as environmentally responsible as industry when it comes to accepting the tenet that cleanup is part of the job.
Environmental impacts and risks
Effects of existing and future regulations on impacts
Publicly owned treatment works receive waterborne wastes, generally from a mix of household and industrial sources, and treat the wastes so that they can be discharged into appropriate receptors with concentrations of all pollutants within prescribed limits.
Treatment plants generally work in two stages. "Primary" treatment mechanically separates the coarser solids from the water, generally by screening and settling. "Secondary" treatment uses oxygen from the air to drive aerobic digestion of the organic wastes by microorganisms. The products of secondary treatment are a volatilized fraction that escapes into the air (generally uncontrolled), a liquid with much lower oxidizable organic content than the incoming waste stream, and a solid fraction that is separated out from the liquid (often by pressure filtration).
The products of the settling tanks used in primary treatment, in addition to the water that is sent on to secondary treatment, include material that has settled to the bottom of the tanks ("primary sludge"), as well as material such as oil and grease that is skimmed off the surface of the tanks. These solids, together with the unrecycled fraction of sludge produced in the secondary treatment tanks ("activated sludge"), are typically digested using an anaerobic process that produces methane, along with lesser amounts of other components such as hydrogen sulfide. In many plants, this offgas is burned to help supply process energy needs, such as process heat for the digesters. The digested solids, termed "biosolids", are often used as fertilizer, or are landfilled. In some plants, the solids are incinerated.
After secondary treatment the water may be released to a natural receiving body, or may be used for irrigation. Depending on the situation, it may be disinfected before release, often by chlorination, or, less frequently, by ultraviolet irradiation. It may also undergo additional filtration, such as through a sand filter, before release.
POTWs are in the majority of cases organized as agencies of local government (often as a department within a city or municipality, but in some cases as independent agencies or authorities). Their operations are funded out of usage fees and tax revenues. Capital funding for major construction projects can be raised through bond issues, typically requiring voter approval. A federal program, the Clean Water State Revolving Fund (CWSRF), established to help local systems comply with the Clean Water Act, has provided nearly $30 billion in low interest loans for projects throughout the country.
As mentioned in the summary section above, a truly comprehensive appraisal of the impact of the POTW sector would include a comparison of the present effluent stream with that which we would be facing in the absence of its effects on industrial discharges. In that sense, through both the destruction or removal of materials from the effluent stream, and the regulation of industrial discharges via the National Pretreatment Program, the impact POTWs on the environment would have to be considered overwhelmingly positive.
However, impacts are generally not computed with respect to hypothetical, and unquantifiable, worst-case scenarios. Fair or not, the impact imputed to POTWs must be measured by how much of each material they deliver to the environment, the fate of that material, and the associated risks.
Aggregate figures on total discharges from POTWs are not easy to come by. According to one published figure, the total discharge of treated water from POTWs is 26 billion gallons per day.
As far as mitigation of impact is concerned, a 1991 EPA estimate cited in a description of the National Pretreatment Program indicated that "190-204 million pounds of metals and 30-108 million pounds of organics "were removed each year as a result of pretreatment program requirements". How much of that total was due to physical removal, and how much to material that would otherwise have been discharged, is not specified, and the uncertainty in the total, particularly for organics, is large. However, the estimate does suggest an order of magnitude -- if the pretreatment program is responsible for reductions in the neighborhood of tens of percent in the total loading from industry, we are probably looking at something in the range of a hundred million to a billion pounds per year as the total discharge of metals and organics in the liquid and solid releases from industry into wastewater that would be treated by a POTW. To see if this is a reasonable ballpark estimate, we note that the 26 billion gallon per day total discharge cited above works out to 9.5 trillion gallons per year, or 79 trillion pounds of water. A hundred million pounds over 79 trillion implies a discharge concentration of about 1.3 ppm for combined metals and organics due to industry. Depending on the metal, typical NPDES limits for POTW discharges would range from fractions of a ppm for more toxic metals to a few ppm for iron and similar less problematic materials. An 80-90% removal rate during treatment would bring the effluent from the POTW into this range, so the estimate seems like a reasonable rough guide. A source of aggregate national data, similar to those available for air pollutants and hazardous solid wastes, would be useful.
Air emissions data for certain key criteria pollutants (ozone precursors) are available from the National Emission Trends (NET) database (1999), and hazardous air pollutant emissions data are available from the National Toxics Inventory (NTI) database (1996 is the most recent year for which final data are available). For POTWs, which are classified under SIC code 4952, Sewerage systems, the total emissions are:
The VOC emission from the sector, while significant, is not particularly intense on a normalized basis, given the large number of facilities and the amount of material flowing through POTW treatment processes. It is roughly comparable to the total amount generated by all U.S. manufacturers of metal office furniture, for example, or of plastic bottles, both of which generate more intense emissions from a much smaller number of facilities. The NOx emission puts the POTW sector somewhat higher among all four digit SIC codes, possibly a reflection of the power needed for compressors, and of combustion associated with boilers. The process heat needed for water treatment is not intense, but it is spread over large volumes of material.
The risks from the discharge of treated water from POTWs are, for the most part, well-understood, and have been incorporated into the accepted practices for deriving NPDES permit limits for POTWs. The risks of air emissions are, except in extraordinary cases (such as a notorious hexane discharge in Louisville KY in 1981 that ignited and destroyed three miles of sewers) of less concern to the general public (though still important to treatment plant personnel). In contrast, the risks of the solid products of wastewater treatment, though they have certainly been intensively studied, must be regarded as much less settled.
By strongly endorsing the reintroduction of municipal sewage sludge into the human food chain, EPA has embarked on a course of action whose outcome cannot be predicted until the consequences have become overwhelmingly apparent. The heavy hand of politics has played and will play a major role in decision-making at every level. This is not necessarily "wrong" in the civics sense. The interplay of contending forces is an ineluctable part of the democratic process. However, it is not conducive to objectivity, nor does is guarantee that "the whole truth" will even be available for objective consideration.
The purpose of this section is not to evaluate the effects of encouraging the application biosolids to the food chain -- it is to evaluate the risks of doing so. Here, the conclusion is unequivocal: ignorance increases risk. There is always a temptation to avoid looking into immediate problems, and thereby put a worse resolution further into the future. Surely the prudent course will be for EPA to challenge aggressively its own assumptions about safe contaminant levels, and to monitor carefully the effects of biosolids application on both the long-term health of the soil and the quality of the produce.
It is reasonable to assume that the agency will maintain an active program, at some reasonable level of activity, to keep track of the developing knowledge base concerning the effects of biosolids application on soil and food. However, it is less certain that such a program will have a sufficiently broad mandate to address the points of greatest concern. Assuming the initial science supporting the present maximum contaminant levels proves valid, the real risks associated with biosolids use in the food chain are not likely first to become apparent in carefully monitored test plots. Tens of thousands of separate experiments are going to be performed over the next several decades on plots of land all over the country. The experiments will test not only the consequences of applying biosolids within the prescribed limits, but will also test the ability of thousands of individual municipalities to manage their wastes within those limits. In many cases, the very municipalities most likely to have difficulties remaining within the maximum contaminant levels will be those least likely, for political reasons, to be subject to close scrutiny. This fact of life can only serve to increase risk, and only a degree of determination not typically apparent in the absence of strong public pressure can serve to mitigate it.
Discharge of treated water from a POTW into a receiving body is regulated under the National Pollution Discharge Elimination System (NPDES). Industries that discharge their effluents directly into receiving bodies such as rivers or other natural formations are also regulated under NPDES rules. However, industries that discharge into collection systems served by a POTW ("indirect dischargers") are regulated most directly by the POTW. It is the POTW's responsibility to ensure that the combined discharges from all the industries connected to its collection system will not cause the POTW to violate its own NPDES permit. The POTW is thus a key link in the regulatory chain.
A completely decentralized system would lead to drastic differences in the rules governing similar industrial processes located in different municipalities. Accordingly, not all decisions are left up to the discretion of the POTW. For those industries whose processes have the most significant impact on wastewater, EPA has established nationally applicable Effluent Guidelines. (Information on both existing guidelines and upcoming guidelines is available on the web from EPA.)
Ideally, POTWs should only handle wastewater that has been deliberately released by specific users connected to the system. However, many collection systems allow water from rain or other natural sources into the flow, either by design (for older, "combined" systems that mix storm and wastewater collection networks), by unintentional cross-connections, or by leaks into the system. Surges of water during storm events can cause treatment plants to be overwhelmed by excessive water volume, and can lead to the discharge of improperly treated water. Considerable regulatory attention has been devoted to devising a system of incentives and penalties to mitigate the effects of stormwater overloads. Stormwater discharges are regulated through a POTW's NPDES permit. Recent changes in the rules (the Stormwater Phase II Final Rule of 1999) have extended regulations that previously applied only to larger POTWs to include smaller systems as well.
A NESHAP regulating emissions of hazardous air pollutants (HAPs) from POTWs, applying to new and reconstructed treatment plants, was published in the Federal Register in final form in 1999. The rule specifically calls out several volatiles of limited solubility in water that are subject to air stripping during secondary treatment. They include aromatic (benzene, toluene, ethylbenzene, xylenes, and naphthalene) and chlorinated (methylene chloride, chloroform, and tetrachloroethylene) compounds that are often found in industrial effluent. They are also present in many consumer products that can be found in domestic sewage, but the regulation applies specifically only to those POTWs that receive discharges from industrial sources. MACT standards from sewage sludge incineration were also proposed in 1997, but were delisted in 2002, apparently on the grounds that HAP emissions from that source were not significant.
The current system allows the application to land of sludges containing persistent toxic materials. While the rate of application is limited by the regulations (according to several different options), there is apparently no limit to the total amount of sludge that can be applied to a given tract of farmland (other than a requirement that the amount not exceed the crop's nutrient needs). However, according to the EPA's risk assessment for biosolids, the concentration of inorganic toxins that cropland can absorb without those toxins showing up at deleterious levels in the crops is, to put it bluntly, astonishing. The risk assessment helpfully provides a table (on page 82) that indicates the quantity of material, for several common toxic inorganics, that soil can contain, in kilograms per hectare, before adverse effects will start to show up in a person consuming crops grown in that soil for a substantial portion of his or her diet. In the table below, the quantities have been converted into soil composition percentages, assuming that the material is concentrated in the top 15 centimeters of soil. (Elsewhere in the risk assessment, data are cited that indicate strong binding by biosolids -- this provides a safety factor for plant uptake rate assumptions, but also implies that, once applied, those elements will pretty much stay put.) Quantities have been converted to pounds per cubic feet, so the amounts of toxin in the reference "safe" soil can be more easily visualized. A slab of soil 15 cm thick and one hectare in area occupies about 53,000 cubic feet, and a cubic foot of soil weighs about 80 pounds or somewhat more, depending on soil type (i. e. soil weight works out to 2 million kg/ha-15 cm, or 1000 tons/acre-6").
Table. Maximum "safe" contaminant levels for agricultural soil,
per EPA biosolids risk assessment.
|
Toxin |
"Safe" quantity (kilograms per hectare)* |
"Safe" quantity (pounds per cubic foot) |
"Safe" concentration in soil (%) |
| Arsenic | 6,700 | 0.3 | 0.3% |
| Cadmium | 610 | 0.03 | 0.03% |
| Mercury | 180 | 0.007 | 0.01% |
| Nickel | 63,000 | 2.6 | 3.2% |
| Selenium | 14,000 | 0.58 | 0.7% |
| Zinc | 16,000 | 0.66 | 0.8% |
*Quantities transcribed from EPA biosolids risk assessment, Table 10, page 82, "Biosolids Risk Assessment Results for Land Application", for Exposure Pathway 1. See Box 9, page 62, for an example of the calculation of the risk of an inorganic pollutant for Pathway 1, "an adult person ingesting crops grown in biosolids-amended soils". Values quoted are for the parameter RPc, "the cumulative amount of a pollutant that can be land applied without adverse effects from biosolids exposure via the pathway evaluated", and is expressed in kg/ha (see for example page 63, bottom).
In other words, one could grow crops in soil that was 3% nickel salt, with each cubic foot containing over a half a pound each of selenium and zinc, about five ounces of arsenic, half an ounce of cadmium, and about ten drops of mercury, and consume the results with perfect confidence.
Of course, the limits were developed for the metals considered separately, and this hypothetical stew has them all combined, but not to worry. Synergistic effects do not compromise safety when it comes to biosolids in agriculture -- they enhance it. An abundance of the less toxic zinc, for example, will actually inhibit the uptake of the more toxic cadmium. EPA is not making this up -- this really happens in the magic world of biosolids. There is no reason to doubt that, at least under the assumptions of the risk assessment, crops grown in the above mix could be eaten with no ill effects.
That is the direction in which the present regulatory regime is taking a substantial fraction of the nation's farmland. To be sure, EPA does not expect any land to be loaded anywhere near the reference "safe" limits calculated above. Somewhat lower limits (ranging from 17 kg per hectare for mercury to 2,800 kg per hectare for zinc) have been established as cumulative maxima for biosolids contaminated up to a ceiling level, and application of biosolids with concentrations near the ceiling limit cannot exceed these cumulative limits. However, for biosolids meeting a somewhat lower concentration limit, there is no cumulative limit specified, and no requirement even to maintain a cumulative record of how much has been added. EPA estimates that it would take 100 to 300 years of biosolids application to reach the cumulative limits -- under normal circumstances. The risk assessment is indeed conservative in the factors it weighs. It does not, however, attempt to assess the conscientiousness of treatment plant operators, or the motives of farmers for whom acceptance (or at least non-refusal) of waste might prove more advantageous under some circumstances than maximization of crop yields.
The bottom line: supporters of land application of biosolids have bought a relatively inexpensive solution to the problem of sewage sludge disposal, and are even collecting a dividend of beneficial reuse of what would otherwise be a high-volume, inexorably accumulating waste stream. The price is eternal, and conscientious, vigilance. That responsibility falls, of course, most directly on the POTWs who are the "manufacturers" of biosolids, and who are responsible for exercising quality control over the material shipped by their "suppliers". But the ultimate responsibility rests with the EPA. The agency has bought heavily into this farm, and will be held accountable for its management.
The sleeper in this situation is the public response. Land application of biosolids has been endorsed by several prominent environmental interest groups (to the frank dismay of some other groups, which are, to be sure, somewhat smaller and less influential). While an analysis of the motives of the regulatory agencies (to supplement the consideration of the consequences of their choices) is fair game for this document, a similar analysis of the motives of environmental groups might take us too far afield. However, we might speculate that, by injecting biosolids literally into the human food chain, the public interest groups supporting application of biosolids to croplands are ensuring that the issue will never fall very far off the public's radar screen, and can be brought to prominence in a heartbeat.
Shapers of regulatory policy who are evaluating potential future scenarios that may develop out of the biosolids saga should understand the roots of the "organic foods" ethos. There is a misconception, reflected for example in typical media presentations of the subject, that the driving force behind organic agriculture is based on health risk. It will be a long time before any beneficial effects of organic food consumption show up in the epidemiological data (if they ever do). But that is a red herring. Organic agriculture, as a "movement", is primarily an ecological and social phenomenon. And it is especially the ecological aspect that is most at odds with the current approach to biosolids disposal. The most telling criticism of "factory farming" is that it turns soil into a passive support medium for crop growth, rather than part of a long-established ecological web. This criticism is generally applied to the practice of adding chemical fertilizers and pesticides -- well-characterized materials whose properties are known and whose concentrations can be carefully controlled and monitored. If public opinion should shift in the direction of questioning the long-term sustainability of modern industrial farming practices (and there are indications that this is happening in some places), then the issue sharpens. Relying on commercial fertilizers and pesticides involves a calculated risk that technology will keep up with unexpected consequences. With biosolids, the calculated risk starts looking like a leap into the unknown. Answering the public's questions in such a context will not be facilitated by a risk assessment, but will require an extensive database summarizing the measured consequences of biosolids application on many sites over long periods of time.
In this regard, the fate of the organic food labeling rules proposed by the U. S. Department of Agriculture should serve as additional motivation. There was strong pressure from a number of interests with a stake in agricultural biosolids use to include biosolids as acceptable soil amendments for produce that can carry a label designation as "organic".
An interactive schematic diagram of a typical wastewater treatment plant can be found on the WEF website at http://www.wef.org/wefstudents/GoWithFlow/index.htm
National Pretreatment Program:
Clean Water State Revolving Fund (CWSRF):
Existing Effluent Guidelines are published in the Code of Federal Regulations, Title 40, Chapter 1, Subchapter N, available at http://www.epa.gov/docs/epacfr40/chapt-I.info/subch-N.htm
EPA provides an index page to existing and upcoming effluent guidelines at http://www.epa.gov/ost/guide/
The EPA Biosolids page provides access to a large selection of background material and guidance documents, at http://www.epa.gov/owm/mtb/biosolids/index.htm . Specific documents used as sources for this analysis include:
Numerous additional links to biosolids information (the "pro" side) are available through the National Biosolids Partnership website (the partnership includes AMSA, WEF, and EPA) at http://biosolids.policy.net/
For the "con" side of the land application of biosolids debate, there are some well-researched and interesting documents available on the web (in addition to the predictable fulminations). Examples include
Soil density calculations can be found at http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec5/Lec5.html