Under current conditions and regulations, spreading sewage sludge on San Joaquin Valley agricultural fields is the cheapest way for large Southern California cities to dispose of their municipal waste.
The viscous, black cake adds free organic matter and fertilizer to poor soils, making them productive and profitable. However, careful use of sewage sludge, also called biosolids, is necessary to ensure pathogens, nutrients and heavy metals do not contaminate groundwater.
“Like any human activity, nothing is completely fail safe,” said Blake Sanden, University of California Cooperative Extension Kern County farm advisor. “Many people wonder why we should deal with these wastes here in Kern County if they are not generated here. When you're dealing with something that smells bad, looks bad and comes from a place nobody wants to talk about, it can generate very strong feelings. People just don't like the concept.”
Sludge is the material scraped from the bottom of chlorinated settling tanks at sewage treatment plants. It is pumped into digesters that usually use anaerobic microbes to further decompose the sludge and pathogens. After leaving the digester it is spun in centrifuges to get to 20 to 27 percent solids cake.
Typically, farmers use biosolids to grow cotton and crops for animal feed. Food crops that don't touch the ground and are harvested at least six months after application, such as wheat, also may be grown with biosolids. Sludge is unpleasant to work with and attracts flies. But despite the product's limitations, because some valley soils are so poor, biosolids present the only economically feasible way to farm the land.
“For some of these operations, the landowner makes more money off the hauling contracts than the farming operation,” Sanden said.
Most studies report beneficial results in terms of crop yield and soil tilth with sewage sludge. However, biosolids researchers have also seen adverse crop reactions.
“I personally have seen major losses in four cotton fields in Kern County,” Sanden said. “But the failures were followed by a good crop the next season.”
Sanden has conducted four years of field trials in Kern County to study biosolids use. Based on his findings, he suggests farmers need to use less sludge on agricultural fields than has been calculated by industry practitioners to avoid potential nitrate contamination of groundwater.
At issue is the rate at which the nutrients go through a chemical transformation that makes them available to the plants, a process called “nitrogen mineralization.” If the nitrogen leaches out of the rootzone before a plant can take it up, it can potentially seep into and contaminate groundwater.
Biosolids typically have from one-half to 1 percent ammonium nitrogen, which, like mineral fertilizer, is quickly available. Two to 4 percent of the materials are organic forms of nitrogen.
“That's the wild card,” Sanden said. “How quickly is organic nitrogen being converted to mineral nitrogen that plants can absorb?”
The figure used by industry is a mineralization rate of 20 percent. However, Sanden said for Kern County conditions and typical agronomic practices, that's too low. If growers count on 20 percent mineralization, more biosolids will be added than necessary to meet the plants' fertilization needs.
“I've found mineralization to be extremely variable, from as low as 3 percent up to 74 percent,” Sanden said. “The average over time and different fields is about 30 percent.”
The 30 percent mineralization rate is more conservative, but not an engineering standard.
“This is a biological process, so you can't make an engineering standard,” he said. “Unfortunately, in the eyes of some contract applicators, that means less sewage sludge can be applied onto a given field. The whole spirit of the enterprise is not cheap waste disposal, but beneficial agronomic reuse. Farmers must use biosolids judiciously to avoid a negative environmental impact.”
Heavy metals are also a concern when using biosolids in agriculture. Metals rinse down to sewer plants from old pipes and industrial wastes. Although businesses in urban areas are now required to have point-of-source treatment before such wastes enter sewer lines, all metals are not stopped.
Heavy metals binding
“Heavy metals will lock up in the top one or two feet of soil if they have a chance to bind to soil,” Sanden said. “However, some organic molecules that do not bind to soil can carry some forms of metals with them, or they can leach through large soil pores, and reach groundwater.”
Other contaminants that can be found in biosolids are perhaps the most troubling to the human psyche. Pathogens, pharmaceuticals and pesticide residues could make their way through the sludge treatment process, onto farmland and potentially into groundwater.
The immediate concern relates to adverse health impacts on workers or the public who may ingest pathogens or suffer ill health brought on by exposure to metals or other chemicals. During the transportation and application process, workers do come into direct contact with the material.
“Many anecdotal complaints of ill health from biosolids applications have been recorded over the last two decades, but clear clinical linkage has not been established,” Sanden said. “Health studies of sewage treatment plant workers and field workers dealing with the application of biosolids have yielded mixed results.”
Governments throughout the valley are wrangling with ordinances to ensure public safety while affording farmers the economic opportunity to use this material and providing Southern California cities this avenue to dispose of wastes.
“Most people want a simple answer: is it good or is it bad. The answer is not that simple. It is not completely risk free, but it has benefits. Just like driving a car,” Sanden said.