James 0. Evans US.Department of Agriculture Washington, D.C.
The soil as a resource renovator “That which is used develops-that
which is not used wastes away” Hippocrates, 4th century
H
orticulturirts, foresters, and agronomists view briars, bush, and weeds not just as nuisance vegetation but, more generously, as plants growing out of place-plants occurring where not wanted. Goats, however, reveal no intellectual bias among plants, and, as they pursue their hungry objectives, do not pause to segregate weeds from their bluegrass and clover neighbors. An analogy can be drawn concerning certain discarded residues and efffuents. By way of illustration, an earth specialist-ecologist or a soil scientist-sanitary engineer, in contrast to most disposal engineers, can be expected to view treated sewage and many other wastes not as refuse but as valuable resources which happen to be at the wrong place, or in the wrong form, or in the wrong amount, or at the wrong time. Indeed, all resources,
whether animate or inanimate, are of limited value to man unless activated and properly utilized. For example, long ago the observation was made that muscles should be used for them to develop; otherwise, they atrophy-a wasteful process. The value of human excreta and other animal wastes has been recognizedif not understood-for many centuries. Historically, the Chinese and other early civilizations applied human organic wastes to the land to improve or maintain soil fertility. And there are numerous documented accounts of the common use of “night soil“ in European countries. The following quote taken from Victor Hugo’s immortal Les Miserables, published in 1862, is remarkably accurate, descriptive, and upto-date :
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Costs of disposal methods for activated sludge Drying and sale as fertilizer
Wet air oxidation (Zimpro)X
Dewatering and incineration‘ Digestion a n d permanent lagoons Digestion and reclamation
of f a r m lands Digestion and reclamation of strip mines I
I
$10
I
‘preceded by high rate digestion Source: Metropolitan Sanitary District of Greater Chicago
‘32 Environmental Science & Technology
I
I
$20 $30 $40 Cost per equivalent dry t o n
$h
1
$60
B.C.
Do you know what these piles of ordure are, collected at the corners of streets, those carts of mud carried offat night from streets, the frightjul barrels of the nightman, and the fetid streams of subterranean mud which the pavement conceals from you? AN this is a flowering field, it is green grass, it is mint, thyme, and sage; it is game, it is cattle, it is the satisfying lowing of heavy kine; at night it is perJumed hay, it is gilded wheat, it is bread on your table, it is warin blood in your veins, it is health, it is joy, it is life.
Little can be said to supplement such eloquent testimony concerning amaterial that many people usually describe with a single, common, four-letter word. A more ancient quote from the Old Testament, Eccles. 1 : 7, is instructive: AN the rivers run into the sea. Yet the sea is not full; unto the place from whence the rivers come, thither they return again. This succinct and poetic description of the hydrologic cycle-the eternal reuse of water-illustrates a well-known but little appreciated fact that nothing is utterly consumed or wasted. Instead, substances simply change from one form to another and are recycled. The law of the conservation of matter not only is true-it must be lived by. An understanding of what this implies suggests that every effort should be made by man to: develop economically feasible means of salvaging waste materials for reuse as salable products; isolate particularly obnoxious, toxic, or otherwise troublesome industrial wastes at their origin so that they may either be subiected to neutraliz-
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ing treatment or converted into useful products; put the wastes to work as sources of energy. The soil-a multifunctional system
AU of our metals are derived from the earth, and the earth mantle is essential to the existence of land plants and animals. Any soil which promotes the growth and well-being of plants must he amply supplied with air, water, nutrients, and living soil organisms. A given soil may serve in a variety of role-as a receptacle or absorbing agent, a storehouse or reservoir, a screening or filtering agent, a purilier or renovator, a vehicle for transmission and recharge, or as a foundation or supporting agent. Furthermore, a body of soil conceivably may serve each role simultaneously. Consider only one role for the moment. From the beginning of life on this planet, soil has served as a cleansing and renovating agent. It is no wonder (and no accident) that the microorganisms responsible for biological purification (Le., degradation and stabilization) in sewage treatment plants have soil as their common origin. Water recycling- a natural phenomenon
As implied by the hydrologic cycle, all water eventually is recirculated and reused hy various processes and organisms. So-called waste waters, including all the dissolved and undissolved substances they harbor, are likewise recycled, gradually by nature and chance or more rapidly by man and design. Indeed, “waste water” treatment and reuse differ from naiural water recycling onlyin dissimilaritiesin the rapidity and intensity of the time-treatment processes involved, the former being
uyray.
1
nrre-yrur
W w l r WU‘rr
rrrrguuun rcvrs u, rennsyyrvunm arere vnrversrry
showed significant increases in forest growth with no harm-.’
subjected to a compressed timetable. In our modern, extremely complex society-largely urban and industrial in orientation-no single waste treatment or disposal process is best suited to all conditions, and no perfect system will ever be devised. It is possible, however, through known treatment plant techniques to renovate completely waste waters into potable water. Advanced waste treatment processes have been developed and are being used by the Federal Water Quality Administration to allow effluent discharges from which absolutely no pollution would enter our surface or groundwaters. There is a cost consideration, of course, and proper disposal is required of the quantities of organic and inorganic sludge and ash materials separated from the emuent water during treatment.
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Through treatment, sewage can be separated into two major components: so-called sludge “solids,” an inorganic and organic fluidized sludge material, almost all of which can be useful for fertilizing and soil conditioning, and a liquid or water fraction. This liquid is not pure water, of course, and contains an appreciable amount of dissolved nutrients which also are available for plant use and growth. Not without reason, this nutrient content is viewed as excessive and undesirable when discharge to streams or lakes produces profuse aquatic growth and the resultant oxygen depletion, fish kills, and hastening of the eutrophication of lakes. And, until recently at least, most of the attention concerning treated sewage has been focused on the liquid portion. Furthermore, the importance of the
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Volume 4. Number 9, SeDtember 1970 733
Sludge application increases corn yields Weekly rate, inches/acre
,
Total applied, tons/acre
0
0
'/4
5.4
YZ
10.8
