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(5) The general conclusions reached as the result of the investigation and consideration of the matter by the Federation of Sewage Works Associations were as follows : (a) Sewage sludge is of use as a fertilizer and soil conditioner, if properly prepared and applied. Its value varies with the type of sludge. (b) Fresh sludge is seldom used and may be troublesome because of grease content, odors, and bacterial content. (c) Digested sludge, whether wet, air-dried, or heat-dried, appears to have application as a substitute for manure and is of about the same fertilizing value. In its wet form it must be used relatively near to the sewage treatment plant. In its air-dried form it may be worth while to transport it a short distance, perhaps 20 miles. In its heat-dried form, with a sufficient nitrogen content, it may justify a considerable haul. The nitrogen content, however, is relatively low (0-5 to 3 per cent, dry basis) as compared with heat-dried activated sludge (3 to 6 per cent, dry basis). (d) Heat-dried activated sludge appears to have real value as a source of organic nitrogen fertilizer, whether used directly or in mixture. It is free from weed seeds, blends quickly with the soil, when applied has a slight odour for a very short period, and it can be used with safety in fertilizing practically all crops requiring nitrogen in organic form. (e) The sewage treatment plants offering digested sludge are scattered all over the United States. The air-dried sludge is most commonly used locally within an easy distance of truck haul. Most of the plants give away this sludge ; many load it free of charge. (/) Air-dried digested sludge should preferably be disintegrated or ground to remove lumps before distributing for use. When so prepared, a charge is usually made in addition to a charge for bags. (g) Both wet- and air-dried digested sludge should be handled like manure and plowed under as soon as practicable after distribution. (h) It would appear that the heat drying of digested sludge depends for its success on local conditions such as the soil and the crops, and on the effort and skill of the sewage works manager in exploiting the product, as well as on its analysis. The cost of equipment and operation is a deterrent to such procedure at small plants. Most of the heat-dried digested sludge is used directly in farming or gardening. A small amount is bought in bulk by mixers. Heat-dried digested sludge can be used with safety at any reasonable time. (i) The limited experience at RoekfOrd, 111., indicates that the enrichment of heat-dried digested sludge to compete with heat-dried activated sludge does not appear to be practical. (j) Digested activated sludge apparently falls into the general digested sludge classification when judged by its analysis and use. (k) Heat-dried activated sludge appears at present (1946) to have a ready market, which the plants at Milwaukee, Chicago and Houston are unable to satisfy. This type of sludge sells retail in bags (by the bag, ton or carload) and wholesale in bulk (carload) at a somewhat lower price, determined by the market price of its organic nitrogen and phosphoric acid contents. (I) Heat-dried activated sludge is of value in the fertilizer and agricultural field for its organic nitrogen and phosphoric acid as well as its humus and its effect as a conditioner in mixed fertilizers. With a nitrogen content of around 4 per cent, or higher, it may justify a rail haul of 1,000 miles, more or less, in carload lots. In bags, it has been shipped 2,000 miles in limited amounts. (m) Where organic nitrogen is required in agriculture, heat-dried activated sludge can be used with safety at any time. (ft) The cost of dewatering and heat drying activated sludge on a small scale appears to have deterred those in charge of the smaller sewage treatment plants from following this practice. (o) With the exception of the Southern California citrus orchards, no organized handling of airdried digested sludge from a number of adjacent plants has been found. A large number of plants, however, both large and small, have developed a market for their entire output. There is no uniformity of charge or of success in disposing of the sludge. In many small plants a knowledge of the uses for sludge appears lacking. It is hoped that this manual will fill the need for such information. (p) Heat-dried activated sludge is the least odorous and fresh sludge the most odorous of the various sludges. The degree of odor from digested sludge may depend on the completeness of digestion. It is therefore desirable that all air-dried or fresh sludge be turned into the soil as promptly as possible. (q) From the hygienic standpoint, heat-dried activated sludge and heat-dried digested sludge appear safe for any reasonable use in agriculture or horticulture." Digested sludge, air-dried, appears safe for such purposes if used like manure and plowed in, when preparing for a crop, and if care is taken not to apply such sludge thereafter on root crops or low-lying leafy vegetables which are eaten uncooked. Thorough digestion and air drying, as well as storage of the air-dried sludge, afford a sufficient protection of health. Up to date, no case is known of sickness traceable to the use of digested sludge or activated sludge. Most of the State health departments, however, advise that sludge (unless heat-dried) should not be used for crops which are consumed raw. Liquid sludge from the digesters should be used with care as it may contain pathogenic organisms ■depending on its age. It is not so safe as air-dried digested sludge. Fresh sludge in the form of liquid or press cake should be regarded as only one degree removed from night soil and treated as such, being used only on forage crops and then plowed in promptly after application.

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