Biosolids seen as fertilizer alternative

The all-time high cost of fertilizer has Alabama producers looking at alternatives. One of these is biosolids from municipal waste water treatment plants.

At one time, the term “sewage sludge” was used to refer to all byproducts of waste water treatment. In fact, Alabama Cooperative Extension had a 1991 circular ANR-607 entitled, “Land Application of Sludge.”

The term biosolids was introduced in 1991, mainly to designate those sludges that were suitable for beneficial use on the land. Britannica Concise Encyclopedia (2008) defines biosolids specifically, “For use as a fertilizer in agricultural applications.” The EPA defines biosolids as “. . . nutrient-rich organic materials resulting from the treatment of domestic sewage in a treatment facility. When treated and processed, these residuals can be recycled and applied as fertilizer to improve and maintain productive soils and stimulate plant growth.”

In 2002, approximately 60 percent of all municipal biosolids were being land applied for beneficial use. An Alabama survey of 61 municipal wastewater treatment plants found that 89 percent of Alabama biosolids were being land applied (Hankins, 2004). Agricultural lands received 52 percent, and 37 percent was used to reclaim mined land.

There is a tremendous amount of variability in municipal biosolids. This has to do with the sources of biosolids (industrial versus residential), the wastewater treatment process, and the dewatering process. Storage time before land application also can make a difference in nutrient concentration.

Waste water may be anaerobically digested, aerobically digested, lime stabilized, or heat treated. Water can be removed by a filter, centrifuge, or gravity.

Some biosolids may be composted to remove pathogens, but most are chemically treated with quick lime (CaO) to raise the pH and generate heat. Others may be just heat treated.

Regardless of the treatment process, EPA’s Title 40 Part 503 of the Code of Federal Regulations basically recognizes the following categories of municipal biosolids that may be land-applied based upon the level of pathogens in the biosolids:

• Class A biosolids are essentially free of pathogens prior to land application. The metal contents requirements under the Part 503 Rule are the same for Class A and Class B biosolids.

• Class B biosolids may have low levels of pathogens which rapidly die off when applied to soils, essentially becoming pathogen-free within a short period following application when the “Part 503” Rule requirements are followed.

• Exceptional Quality biosolids have lower metals concentration requirements than either Class A or Class B biosolids and have the same pathogen levels as Class A biosolids. These biosolids may be land-applied with no restrictions.

• Biosolids that contain high levels of one or more pollutants or high levels of pathogens may still be land-applied, with certain restrictions that may include (1) Limited loading rate; (2) Detailed records where it is applied; (3) Restrictions to prevent runoff or leaching, increased monitoring of application site(s), etc.

Adding quick lime (calcium oxide) to wet biosolids is perhaps one of the least expensive and easiest ways to reduce pathogens in the biosolids. The quick lime generates heat and a very high pH (greater than 12.0) which effectively kills pathogens.

The resulting biosolids may not be as good for crops as the untreated biosolids, especially if the sludge is held for a while before spreading on the soil surface as in pastures and hay fields.

Lime-stabilized biosolids from an Alabama waste water treatment plant had 30 percent total neutralizing value and was as effective as ground agricultural limestone (63 percent effective neutralizing value) at raising the soil pH when incorporated into a sandy loam soil (Hankins, 2004).

The problem was that the biosolids contained only 0.5 percent total nitrogen (10 pounds per wet ton). When it was applied as a source of nitrogen (equivalent to about 8 wet tons per acre), most of the nitrogen was not taken up by plants, presumably because N volatilized because of the high pH.

Only the Class A biosolids were lime stabilized. Hankins (2004) concluded that lime-stabilized biosolids could be used effectively as a soil liming material, but were not very good sources of nitrogen whether surface applied or incorporated. He did not study P availability in biosolids but conventional knowledge of P chemistry suggests that P availability to crops would change very little when the biosolids are lime stabilized.

Heavy metals (Hg, Pb, Cd, Cr, Zn, Co, Cu, etc.) are often cited as concerns in municipal biosolids. At one time, when heavy industry was allowed to dump wastewater containing metals into municipal systems, this was a serious concern.

Once metals are in the soil, they do not leach out or volatilize like nitrogen. Depending upon soil pH, organic matter, and other factors, some plants could hyper accumulate metals allowing them into the food chain.

Partly because of EPA’s final 503 Rule in 1993, most municipalities now exclude hazardous industrial wastewater from the wastewater stream.

Biosolids that are recycled come from treated wastewater from our own homes. Only a few maximum values exceed the limit for exceptional quality biosolids.

Because so many biosolids are being recycled onto Alabama farmland and reclaimed mine land, some nearby residents are sure to become concerned. Because Alabama counties have little or no rural zoning and private property rights are precious to most rural residents, there is little a neighbor can do if the landowner/applicator is spreading biosolids according to EPA guidelines.

In Alabama, land application of municipal biosolids is enforced by US-EPA Region 4 in Atlanta, Ga., not the Alabama Department of Environmental Management (ADEM).

Odor usually triggers most complaints. Odor problems can be reduced by using best management practices when land applying any organic byproduct. Fortunately, a good rainfall usually dissipates most odors from manure or biosolid applications.

Injection into the soil helps but this is not always possible.

While odors can be unpleasant and annoying, they are not chronic. Furthermore, Alabama landowners have traditionally been protected from such nuisance complaints.

Recently, some Alabama farmers have taken advantage of land application of biosolids from out-of-state sources. While our data show that the nitrogen benefit from lime stabilized biosolids is low, these alternative sources of nutrients are inexpensive and provide our growers with a way to contribute to beneficial recycling of nutrients from our ever-growing urban populations.

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