Sewage sludge is the solid material removed from sewage during the sewage treatment process. It contains plant nutrients, organic matter, and microorganisms and, when properly used, is a satisfactory fertilizer material

McCall, Wade W.

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HAWAII COOPERATIVE ,EXTENSION SERVICECollege of Tropical Agriculture and Human ResourcesUniversity of HawaiiGENERAL HOME GARDEN SERIES No.9SEWAGE SLUDGEby Wade W. McCall*Sewage sludge is the solid material removed fromsewage during the sewage treatment process. It con-tains plant nutrients, organic matter, 'and micro-organisms and, when properly used, is a satisfactoryfertilizer material.TYpes of Sewage SludgeFive types of sludge are produced as follows: (1) raw,(2) digested, (3) activated, (4) digested activated,and (5) chemically precipitated. The types mostoften used as fertilizers are the digested sludge andthe activated sludge.Digested SludgeThe digested sludge is generally produced from amixture of primary and secondary treatment solidsthat are stabilized by a 14- to 55-day heated anaero-bic digestion process.The digested sludge contains 1.5 to 2.2 percentnitrogen (N), 0.48 to 0.88 percent phosphorus,(1.1-2.0 percent available phosphoric acid (P20S»,and 0.17 to 0.91 percent potassium (0.2-1.1 per-cent water-soluble potash (K20». In addition, thedigested sludge contains many ofthe micronutrients.In areas where industrial wastes are added to thedomestic sewage, the digested sludge may be lowerin nitrogen, phosphorus, calcium, and magnesiumand often higher in heavy metals such as nickel,lead, and chromium. These latter materials maycause harmful residues in food crops if sewagesludge is used as a fertilizer source for these crops.The digested sludge may be air-dried or heat-dried.The air-dried material may contain 10 percent ormore moisture and the heat-dried product less than5 percent.The digested sludge may not be in acceptable physi-cal condition for application to the soil. The digestedsludge should be passed through a shredder orgrinder to produce a material easier to handle andto incorporate into the soil. This material may stillcontain viable weed seeds, especially tomato seeds.Digested sludge generally has little offensive odor,although if heavy applications are applied to -thesurface of the soil /and then wetted, a noticeableodor may be detected. Digested sludge is believedto be free of disease-causing organisms. 'Digested sludge is generally available in the vicinityof sewage disposal plants. It is of lower fertilizervalue than the activated form, and for this reasonlittle of it is commercially available for fertilizerpurposes.Activated SludgeThis type of sludge is produced by inoculating thesewage with special microorganisms and aerating itfor several hours. The sludge is filtered off, with orwithout the aid of coagulants, dried in rotary kilns,and then ground and screened. This process resultsin a material that is in excellent physical conditionfor application to the soil. The activated sludge isfree of odor and does not attract flies or othervermin. It contains 5 to 7 percent nitrogen, 0.88 to2.20 percent phosphorus (2 to 5 percent P20S),°to 1.66 percent potassium (0 to 2 percent K20),significant amounts of micronutrients such as zinc,copper, manganese, molybdenum, and boron. Mostof the commercially available sewage sludges are ofthe activated type.Use of Sewage SludgeSewage sludge is primarily a source of nitrogen andmay be used where this plant nutrient is needed. Itis a poor source of phosphorus and potassium andshould be supplemented with sources of these nutri-ents for plant needs. The nitrogen is of the slow-release type, so that it may be applied at any stageof plant growth with little danger of "burning" theplants. Most sewage sludge is used for applicationto turf, but may be used with other crops. It mayalso be added to mixed fertilizers to improve theirphysical condition and to reduce the danger ofburning when the fertilizer is applied.Issued in furtherance of Cooperative Extension Work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Depart-ment of Agriculture. Noel P. Kefford, Director of the Institute and Dean of the College, Cooperative Extension Service, Col-lege of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, Honolulu, HI 96822. An Equal Opportu-nity Employer providing programs and services to the citizens of Hawaii without regard to race, color; national origin, or sex.How to Apply SludgeAdd sewage sludge to the soil at the time of prepa-ration and thoroughly mix it with the soil. For mostgrasses, apply the sludge at the rate of 50 to 55pounds per 1000 square feet of soil area. After theplants are established, fertilize them as often asneeded to maintain the desired green color of thelawn . Apply sludge as a top dressing at the rate of35 to 50 pounds per 1000 square feet. At higherelevations, conversion of the nitrogen contained inthe sludge may be slowed up, thereby decreasing itseffectiveness as a fertilizer material.Digested sludge may contain soluble salts . There-fore, the material should be tested and, if salts arepresent, 'care should be taken in using the material,to prevent damage to seeds or plants. Avoid applica-tion rates above 25 to 30 pounds per 1000 squarefeet, especially"on sandy soils.SummaryWhen properly used, either the activated or thedigested form of sewage sludge is a satisfactoryfertilizer material. Its slow rate of availability greatlyreduces the danger of burning plants, and its use asa fertilizer is an effective means of disposing ofwaste material.*Soil Management SpecialistNOTE: The use of trade names is for the conven-ience of readers only and does not constitute anendorsement of these products by the Universityof Hawaii, the College of Tropical Agriculture andHuman Resources, the Hawaii Cooperative Exten-sion Service, and their employees.Reprinted June, 19So-2M

Sewage Sludge

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Sewage Sludge

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