Iwa international Specialist Conference

23

AGRICULTURAL USE OF SEWAGE SLUDGE - A CONTROVERSIAL ISSUE

H.H.Hahn

University of Karlsruhe

Karlsruhe/Germany

1. Preliminary Remarks
Agriculture has since long fulfilled the role of production on one hand and protection on the other, of innovation on one side and conservation on the other. Agriculture as the oldest activity of settled mankind is in many respects also one of its most noble.
Using residuals from the treatment of spent waters, formerly referred to as sewage sludge and more recently re-named ´biosolids´ for the fertilization and structural improvement of agricultural land also has had a considerable tradition. Very early wastewater collection occurred in many areas with the aim to use this water for agricultural irrigation with the added benefit of using also its constituents. As collected wastewater was not only used for irrigation but re-introduced into the aqueous environment and therefore treated, it was logical to make use of the material separated from the wastewater stream in the course of its treatment.
As the amount of water used by household and industry increased and the intensity of agricultural production called for mechanization and intensive fertilization the formerly symbiotic cooperation of farmer and biosolids producer became more and more obsolete and agricultural sewage sludge utilization was transformed into a convenient type of disposal. – In addition the quality of some sewage sludge changed due to the growing complexity of raw materials, production emissions and household goods that all ended up in those residuals from all anthropogenic activities of settled areas, for in some sense sewage sludge is also a mirror of all of man’s present and former activities.
It is understandable that agriculture in its role of protecting on one hand its products and on the other its resources with which produce is generated hesitates to cooperate in a purely disposal oriented scheme. And it is well advised to shy back from the use of all those residuals that producing and consuming mankind rids itself off. This reflects the present very heated and unfortunately also very emotional discussion of the benefits and disadvantages of agricultural use of biosolids in Germany.
The following remarks are a summary of some of these discussions, trying to maintain a position as objective as possible. The discussion follows the pro and contra arguments listed in the following table 1.

2. Non-discriminating use of all types of secondary raw materials can create risks, also in agriculture.

In our industrial production as well as in our consumptive behavior we have learned to think in cycles as nature has taught us. Yet, after establishing such cycles, for instance in industrial production schemes it became apparent that there must be provisions for getting rid off of unwanted or dangerous material from such closed loops if the inherent accumulation and product impairment is to be stopped.
Past and present experiences with various types of material and energy cycles can be summarized into guidelines for a reasonable use of secondary materials as follows:

• 
  The more closed the cycle is (i.e. next to no input of the material under consideration) the larger the danger of pollutant accumulation.
  
• 
  The fewer the steps are within the use and re-use of raw materials in production is, i.e. the more directly consumption follows production which then is followed again by the same production, i. e the more homogeneous the partners are in such cycles, the more critical is the quality impairment.
  
• 
  The larger the number of additional or production aiding substances is, the higher is the ballast in the recycled material.
  
• 
  The more a product consists of secondary raw material the higher the efforts in stopping pollutant accumulation must be.
  

CONTRA agricultural use of biosolids	PRO agricultural use of biosolids
As it is known from industrial water management: Closing water and other material cycles without pollutant export can lead to dangerous accumulations.	Agricultural use of water and also spent waters: Such recycling of water leads logically to the use and recycling of water constituents.
Agricultural use of biosolids should not be motivated by arguments of cost minimization and convenient disposal.	There are more and more positive examples of the reasonable use of secondary raw materials.
Importing biosolids of “unknown origin” onto agricultural areas can cause specific damages.	Limiting kind and amount of materials that are transported directly or indirectly by (waste)water in order to maintain the option of agricultural use of the residuals of–water treatment is also a beneficial imperative for the overall sewage collection and treatment concept.

Table 1: Arguments for and against agricultural use of biosolids.
We have learned that agricultural use of biosolids can be a very expedient method of cost minimal disposal. However, this is the wrong type of argument: Agricultural use of biosolids should not at all be motivated by arguments of cost minimization and convenience maximization. If we would summarize the past and present experience it would read as follows:

• 
  Historically wastewater constituents have been considered a significant side benefit for agricultural soils if sewage was used for irrigation.
  
• 
  Then, as sewage collection and wastewater treatment became intensified with the very dynamic industrialization for instance after World War II there was definitely a phase of planning and building predominantly wastewater treatment plants without the simultaneous planning and building of sludge treatment facilities.
  
• 
  In some instances this led to an emergency situation in the handling and safe (landfill) disposal of the produced sewage sludge and the pressure on agricultural use/disposal was high.
  
• 
  The subsequent development of sludge handling processes and the construction of treatment facilities have taught us that sludge treatment and disposal can be as expensive as wastewater treatment (or even more expensive). – From that point of view it was understandable that the (economic) advantages of agricultural use were argued.
  
• 
  In the phase of indiscriminant wastewater collection and treatment there were residuals produced that reflected more the complexity of today’s production and consumers spectrum than the promise of nutrients and structural improvement of agriculture. – And consequently there were undoubtedly cases of agricultural use that should not have occurred.
  
• 
  Presently we witness another imparity in this field: the option of thermal use of sewage sludge has been developed to such a degree that the capacities for co-incineration of biosolids and conventional energy carriers are larger than the demand; this causes irresistible offers at low, non-market prices.
  

In some instances of sewage sludge utilization on agricultural areas in the past we have imported non-degradable and unwanted or even dangerous material into these soils. This has caused specific damages. – To be fair, this has not occurred in a scale that would be alarming. But it has occurred either for reasons of carelessness or for the simple reason of seeking short-term benefits.

And it is this type of black-sheep behavior of indiscriminant disposal that has led to the development of a very tight network of regulations and supervision. From this we have learned the following:

• 
  Mass-balances must be done as complete as possible for all substances used (or imported) on agricultural land, where in addition to the input all output and all possible transformations are quantitatively described.
  
• 
  If import of questionable substances leads to loadings that are critical then not only one source of input but all must be scrutinized.
  
• 
  Simultaneously we need information on permissible loadings (for instance specific heavy metals on soils used for specific crops); these permissible loadings or maximum concentrations are obtained from bioaccumulation factors, maximum daily intake factors in (human) nourishment and transfer functions for instance from a given soil to a specific plant and from there to the (human) food chain.
  
• 
  Such data exist in a rather small number of instances; but the existence of those illustrates that a rational argumentation is possible and it also describes the type of information gather necessary.
  

3. Secondary raw materials become more and more valuable, also in water technology.
Very early it was shown even in regions with relative abundance of water that agricultural use of spent waters was necessary. This recycling of water logically led to the use and recycling of water constituents that were removed in the course of pretreatment of irrigation waters as well.
Until very recently statistics on the agricultural use of biosolids in European countries (as for instance published by the European Water Association) show a widespread use of these resources and therefore also a broad basis of experience. This positive as well as negative experience can be summarized as follows:

• 
  There are acceptable and (in part scientific) proven limits or standards for the quality that a biosolids matrix should have so that its application on agricultural soils is beneficial.
  
• 
  These standards are presently re-evaluated in the context of European legislature.
  
• 
  Many sludge streams are of such quality that these limits are not at all reached. In fact there is as continuous improvement of many such sludge in terms of heavy metals content (so that various secondary fertilizers carry a lower load of heavy metals than some virgin mineral fertilizers).
  
• 
  Agriculturally utilized sludge is highly quality controlled and accompanied by advice for the farmer in terms of nutrient content of the fertilizer and nutrient demand of the soil (giving the farmer a much better basis for all his decisions); this frequently leads to a lower application rate of biosolids on soil than would be permissible in general and specifically in terms of heavy metals or pollutants loading.
  

It has been mentioned that biosolids are valuable (secondary) fertilizer materials in addition to their contribution to structural improvement of soil matrices. In general we observe today that there are more and more positive examples of the reasonable use of secondary raw materials. And such use of raw materials becomes more and more beneficial in those instances where the store of primary raw materials begins to be exhausted as well as in those instances where the mining, producing and providing of such materials is transport- and energy-intensive.

Phosphorous, one of the main fertilizing components of biosolids, as such a raw material which begins to be exhausted in some of its natural sites. This has led to the development of recycling concepts.

• 
  There are basically two different notions; one would be the “selective harvesting” of phosphorous from wastewaters in such amount and quality that it can be re-introduced into the overall phosphorous production cycle and, second, the “global” use of the phosphorous-rich sewage sludge as fertilizer (which contains some more fertilizer components in macro and micro fractions as well as the above mentioned potentially polluting materials form other anthropogenic activities.
  
• 
  The “production” of fully re-usable phosphorous is still technically and economically in its development and can as of now not yet compete economically with the import of virginal phosphorous ores (or derivatives of that). – Thus it is reasonable to focus on and support the second concept of a more limited re-use of secondary phosphorous directly as fertilizer.
  
• 
  As mentioned before this concept of agricultural re-cycling has not only had a long tradition with lots of experience collected in the course of its use but has become a vehicle of more rational analyses of farmers´ soils and soil improvement techniques.
  
• 
  Yet, at the same time it must be said that a complete analysis of a specific sludge in terms of all its constituents which might even be technically feasible would be economically unadvisable or impossible. To insure oneself of the acceptable quality of a given sludge might require a complete input-output analysis for a specified region. And this is very much in keeping with the ideas of the new agriculture.
  

One of the only “indirect” yet most significant advantages of agricultural use of sewage sludge is to limit type and amount of materials that are transported directly or indirectly by (waste)water in order to maintain the option of agricultural use of the residuals of water treatment. It has been indicated before that there were cases of inadvertently harming agricultural soils by applying pollutant carrying sewage sludge.

These historical (and rather rare) incidences have led to careful analyses on sources of such pollutants and the control of these sources, besides intensive affords to repair the stated damage. We have learned that these sources can be found with an acceptable effort and we have also managed to control them to the degree needed. Thus, aiming for a potential use of biosolids from wastewater treatment in agriculture is also a beneficial imperative for the overall sewage collection and treatment concept:

• 
  Historically our sewerage system has been developed primarily for domestic sewage; its collection, its routing and the concept of treating at the so-called endpoint all sewage might still be an acceptable solution today if only domestic waste is concerned. – However the introduction of urban runoff on one side and complex industrial wastes on the other, have led to a questioning of the concept of centrally collected, gravity transported, combined treated sewerage.
  
• 
  Our concept of a central disposal system (collection, treatment and re-introduction into the aqueous environment) has formerly led to a dangerous dissipation of all kinds of dangerous and oftentimes anthropogenic pollutants.
  
• 
  With checks on the collection system for the sake of producing agriculturally safe biosolids (for instance by strict standards for wastewater discharge into sewer systems) the overall concept of our sewerage has been improved or at least saved, be it by saving the structural stability of sewers, maintaining the well-functioning of treatments plants and guaranteeing the acceptability of discharged treated wastes and separated solids.
  
• 
  If today we would relax the standards of discharge into the system, or not re-evaluate them continuously in the light of new insights because the ambitious aim of agricultural use of biosolids no longer exists then the overall situation in the operation of infrastructure and pollution control would be serious endangered.
  

4. Conclusions
One must not be very prophetic to predict a significant re-evaluation and re-orientation in the longstanding tradition of agricultural use of residuals from wastewater treatment. And it will be most likely that the total amount of sludge being utilized in this way will become smaller than it is now.
There are two aspects of such agricultural use of biosolids that make it worthwhile to try to maintain this option. First of all such use of secondary raw materials will remind us that there are other input streams as well as output and that we must attempt to obtain an ever more complete picture of all these mass movements in the interest of our own health. And second, quantifying the requirements for good quality biosolids will provide a sound basis for the operation of our inherited disposal concepts using the well-developed infrastructure of central sewerage.

24

The economical challenges towards sustainable sludge management

Dr Tim Evans,

Tim Evans environment, Stonecroft, Park Lane, Ashtead, Surrey, KT21 1EU, England
tel/fax [phone] email: [e-mail]