Practically and philosophically, any discussion of sludge management, particularly the treated sludge from municipal wastewater treatment plants, pivots to a discussion regarding possible uses for the sludge. As early as 1907, it has been confirmed that municipal sludge from Alliance, Ohio was used as fertilizer, much like animal manure. Upon evaluation, in reports dating from 1942 to the late 1970s, the nutrient composition of sludge is considered “similar to other organic waste-based soil amendments that are routinely applied on cropland.”
Existing regulations regarding the use of reclaimed wastewater on food crops are designed to protect consumers from exposure to pathogens and potential health hazards have been addressed, but similar studies have not yet been widely conducted on the use of treated sludge as fertilizer.
It is a fact, though, that sludge has been traditionally used as fertilizer, not only in other parts of the world, but also in unregulated regions of the United States. In addition to nutrients that are essential for plant growth, sludge can also contain trace elements that might help to replenish soils that have been over tilled and are deficient in such elements as zinc, copper and iron. Controlled land applications of element-rich treated sludge could help to remedy such deficiencies.
The World Health Organization has also investigated the possibility of sludge application to cropland, and as noted, the EU currently supports the application of treated sludge on agricultural land.
Irrigation with reclaimed water currently occurs. Both water shortages and waste disposal are driving forces that support the use of reclaimed wastewater, and landscape irrigation a primary and fast-growing demand. Increasing residential development is viewed as a catalyst for removing treated effluent from the spectrum of agricultural use, although that may change with better technology and higher demand for crop irrigation. Economically, cost of reclaimed water is relatively high. If farmers still have access to low-cost water for irrigation, they will not be likely to seek out reclaimed supplies for agricultural use.
Nationwide sludge has minimal impact on agriculture at present. What is true, however, is that sludge, unlike water, can be transported long distance at relatively reasonable rates. In the 1990s, some of New York’s treated sludge was shipped to northeastern Texas and eastern Colorado for use on local cropland, and the state of Massachusetts shipped a portion of its treated sludge to Florida in the form of heat-dried pellets.
It is probable that the debate over agricultural use of treated sludge will continue, perhaps for decades. The pros and cons have now been discussed with no resolution, for at least half a century or perhaps much longer, as it is. The following words were included in the 1996 book quoted earlier. They are still pertinent today:
“If the total amount of municipal sludge produced in the United States were applied to cropland at agronomic rates less than 2 percent of the nation’s cropland would be necessary to accept it. However, there are some regions where limited cropland acreage may constrain sludge management options. A lack of available disposal options near densely populated urban centers has forced many municipalities to seek distant disposal and land application sites at considerable costs. Given these economic and geographic constraints, it is not likely that all of the sewage sludge will be applied to cropland in the foreseeable future, and thus only a very small percentage of the food crops grown in the United States would ever be exposed to sewage sludge.”
While the technology may soon exist to assure that treated sludge is free from toxic substances and pathogens and deemed safe for use on crops just as treated wastewater already is, it may well be that public perception will continue to be resistant to the idea.
Only time will tell.