Long-term Effects of Sewage Sludge and Farm Slurries Applications

J.H. Williams, G. Guidi, P. L'Hermite

• Publisher: CRC Press
• Year: 1985
• eBook ISBN: 9781135473846
• Edition: 1
• Topic: Sciences biologiques
• Subtopic: Science environnementale

Anaerobic digestion has no effect on the quantity of the waste treated, but it affects its quality and its usefulness as fertilizer. This conference proceedings collection of twenty papers delves into the issue.

A.
CUMULATIVE AND RESIDUALS EFFECTS OF SLUDGES AND FARM SLURRIES PART I

Use of digested effluents in agriculture

Soil microorganisms and long-term fertility

Comparison of the efficiency of nitrogen in the cattle and pig slurries prepared acoording to three methods: storage, aeration and anaerobic digestion

Long-term effects of the landspreading of pig and cattle slurries on the accumulation and availability of soil nutrients

Relationships between soil structure and time of landspreading of pig slurry

Results of large-scale field experiments with sewage sludge as an organic fertilizer for arable soils in different regions of the Netherlands

The cumulative and residual effects of sewage sludge nitrogen on crop growth

Long-term effects of farm slurries applications in the Netherlands

Discussion on Part I

USE OF DIGESTED EFFLUENTS IN AGRICULTURE

M.Demuynck, E.-J.Nyns and H.Naveau

Unit of Bioengineering, University of Louvain,
B–1348 Louvain-la-Neuve, Belgium

Summary

Whereas anaerobic digestion has no effect on the quantity of the waste treated, it has well an effect on its quality and consequently on its fertilizer value. Indeed, first of all, 30 to 40 per cent of the organic matter of the waste digested which can be either manure or sludge, are transformed into methane and the remaining organic matter is more stable. If the total nitrogen content of the waste remains more or less the same, the proportion of ammoniacal nitrogen increases (10 to 70 % increasing are reported) and the proportion of organic nitrogen decreases.

The hygienization effect of the anaerobic digestion process is only partial i.e. it allows the destruction of some of the pathogens (bacteria, viruses) under normal running operations. The digested waste is only free of pathogens after treatment under thermophilic conditions.

Concerning the spreading of the digested effluent on land, the effect of the biomethanation treatment is positive since first, the waste is desodorized and secondly, the waste is liquified, homogenized and less sticky and therefore disappears more quickly into the soil which avoids the suppressing of the vegetation and hinders weeds to come out.

Since the disponibility of the nitrogen of the digested effluents is higher than for effluents which are raw or stabilized by other means, yields of crops high demanding in nitrogen will be improved. Ryegrass and maize are typical examples. With the lowering of the C:N ratio of the digested effluent the depressive effect which occurs normally after the application of sewage sludges can be avoided. From all these considerations , recommendations for the utilization of digested effluents are proposed.

1. INTRODUCTION

If anaerobic digestion was already well developed in the past for the stabilization of sewage sludges, this process becomes at present currently applied for the treatment of agricultural and industrial wastes. Most of the anaerobically digested effluents are not further treated at all and are used in agriculture as fertilizer or soil conditioner.

This is especially the case for part of the sludges produced in the European Community and for the largest part of the agricultural wastes digested. If the farmers know well what is the fertilizer value of their untreated manures or other farm residues, there is at present no precise estimation of the fertilizer value of the digested agricultural wastes. If digested sewage sludges are already often used in agriculture, their potential users may not know exactly what type of fertilizer manure they have.

It may be asked if the anaerobic digestion process has an effect on the organic waste, if yes to which extent and consequently what it involves in practice. Does anaerobic digestion modify the composition of the waste? Are the anaerobically digested wastes more free of pathogens than the raw ones? Is their quality improved by the treatment? Are the availabilities of their nutrients better or worst after digestion? Can farmers obtain the same crop yields with the digested effluents than with the raw wastes? Do they have to modify the methods and the times of waste application? Can they use them on all types of crops?

Based on a review of the literature and on known experimental results, a study has been realised attempting to answer these questions. This paper gives the essential conclusions of that study.

2. EFFECT OF THE ANAEROBIC DIGESTION ON THE WASTE COMPOSITION

A general consideration to be repeated here is that the anaerobic digestion or biomethanation process affects only the quality of the waste that is treated and that it does not change its quantity at all. On the contrary, sometimes the volumes of the wastes may be increased; this can be the case for semi-solid or solid wastes which are diluted before entering the digester.

The effect of the process on the quality of the waste will depend on several factors as the biodegradability of the waste, the retention time in the digester and the temperature of digestion. A high content in easily biodegradable matters of the waste, a long retention time and a high digestion temperature may lead to important modifications of the nutrient content of the digested effluent. At the contrary, if the waste is not easily degradable and if the digester is not well operating, there can be practically no modifications at all. It is so that the composition of sewage sludges is in general more affected by anaerobic digestion than the agricultural wastes, especially those containing high quantities of lignin, cellulose, those two organic substances being not or partly bioconverted.

In summary, anaerobic digestion has two effects on the nutrient content of the wastes: first, it decreases their organic content and consequently their carbon content and secondly, it transforms part of the organic nitrogen into ammonia nitrogen. The organic matter can be reduced by almost half for sewage sludges (1). For liquid and semi-solid manures, the organic matter reductions reported are of about 40 per cent (2) (3) whereas for manures with bedding, the reduction is less than 20 per cent (4). The remaining organic matter is more stable than originally.

The proportion of ammoniacal nitrogen related to the total nitrogen content increases from about 5 per cent up to as high as 70 per cent. The mineralizations of nitrogen are again more spectacular for sewage sludges and are more important under heated digestion than cold digestion. This mineralized or ammoniacal nitrogen is more readily available to plants than the organic one. This is thus an improvement of the quality of the treated waste. Nevertheless ammonia can be rapidly lost under certain circumstances; precautions for utilization of the digested effluents have so to be taken so as it will be seen further.

A direct consequence of these two effects is that the carbon to nitrogen ratio decreases after anaerobic digestion. For wastes having originally C/N ratios above 15 (raw sewage sludges, manures with bedding, crop residues), this is thus also an improvement. Indeed, generally after digestion, their C/N ratios decrease to around 10 and so there is no nitrogen immobilization once the waste is incorporated in the soil.

Concerning the other nutrients i.e. phosphorous, potassium and other macro nutrients, they appear to remain unaltered by anaerobic digestion. Nevertheless, since the dry matter content decreases in the digested effluents, their concentrations increase after digestion when expressed to the dry matter content.

The same observation is valid for the heavy metals and other trace elements’ content. Although, these potentially toxic elements are precipitated during anaerobic digestion, it appears that once in the soil, these precipitates will oxidize and so have the same behaviour as those of raw wastes (5). Concerning the degradation of nonionic surfactants that may be present in sewage sludges, it appears that anaerobic digestion favours the production of 4–nonyphenol, a persistent toxic substance. Neverthe less, there is at present no case of intoxication reported. On the other hand, the process appears to favour the degradation of most of the insecticides (7). Although this has still to be verified for full-scale operation, this may be a point in favour of the anaerobic digestion.

3. REDUCTION OF PATHOGENS BY ANAEROBIC DIGESTION

Both sewage sludge and animal wastes may contain a variety of pathogens which present disease hazards to man and food animals or plants. They may be bacteria, viruses, helminths, fungi or protozoa.

At laboratory scale, mesophilic anaerobic digestion allows a significant reduction of those pathogens. Indeed, the relative reduction is good for plant pathogens and parasitic cysts, moderate to good for bacteria, moderate for viruses (12), and poor for helminths ova. This pathogens’ reduction effect is very similar to conventional aerobic digestion. In practice, especially in completely mixed digesters (12) (14), this effect is reduced by short-circuiting and by simultaneous drawing off and feeding of the digester (15) (16). In that case, there is always a risk that the mixed liquor does not remain inside the reactor during the whole retention time; infectious organisms may so be discharged and recontaminate the rest of the effluent. To obtain at full-scale operation the same disinfection effect as at laboratory-scale, the waste should be treated in batch or plug-flow digestion systems. An effluent completely free of pathogens can only be obtained after digestion at thermophilic temperatures i.e. around 55 °C. Even for helminths eggs, there is a complete inhibition of normal egg development (17).

Although thermophilic anaerobic digestion is seldom employed until now, it should be recommended in the future to digest at 55 °C the most infected wastes as abattoir wastes for example.

4. QUALITY IMPROVEMENT OF THE WASTE BY ANAEROBIC DIGESTION

Besides its two effects on waste composition and disease survival, anaerobic digestion may also improve the quality of the waste. First of all, it reduces the odour of the waste. Indeed, both for sludge and for manure, smells during utilization (spreading) cause more complaints than any other aspect of the operation. These problems can be minimized and controlled by using digested effluents. It has for example been calculated that the emission from 1 ha spread with digested slurry is 10 per cent of the emission from 1 ha spread with untreated slurry (18). The reduction of smell could justify the fact that cattle prefer grazing on pastures spread with digested manures (19). Although the odour reduction is not as complete as the one obtained by continuous aeration, the odour reduction by anaerobic digestion is of higher long term value. Indeed for well anaerobically digested manure, odour remains unchanged even after 120 days of storage (19), and the concentrations of malodorous compounds may even be reduced during that time (20). Comparatively, well aerated manure, has been reported to smell again after two to three weeks (19).

As most of the waste treatments, anaerobic digestion reduces the viscosity of the wastes; consequently they become less sticky and more homogeneous (21) (22) (23). Their improved physical state leads to an easier handling and blockages in spreading equipment are avoided. The digested effluents disappear more quickly into the sward and do not form a mat as they dry which smothers new grass. Consequently, there is no or less opportunity for weeds to grow (22). Weed control occurs also in the anaerobic digester itself. Anaerobic digestion appears to have a quite significant fatal effect on weeds as rumex, millet, effect which should be superior to the one of storage or aeration (22). This weed control is nevertheless restricted to the wastes which undergo the treatment and has consequently limitations. It avoids at least the transmission of weeds by manure and other wastes.

5. INFLUENCE OF DIGESTED EFFLUENTS ON THE SOIL

Organic waste may be used as soil conditioner so as to restore derelict and disturbed land e.g. mining spoil, landfill sites, unproductive land…Such soils have very low levels of nutrients and the lack of organic matter can render them more susceptible to physical damage, particularly compaction from heavy machinery resulting in poor root penetration, low infiltration rates with consequent risk of runoff, erosion and pollution (24).

By increasing the carbon content of the soil, o...

Table of contents

• COVER PAGE
• TITLE PAGE
• COPYRIGHT PAGE
• PREFACE
• A.: CUMULATIVE AND RESIDUALS EFFECTS OF SLUDGES AND FARM SLURRIES PART I
• A.: CUMULATIVE AND RESIDUALS EFFECTS OF SLUDGES AND FARM SLURRIES PART II
• B.: CHANGES ON STORAGE AND MINERALISATION STUDIES IN SOILS AFTERTREATMENT
• C. METHODSFOR EVALUATING THE COMPOSITION OF SLURRIES MANURES
• SUMMARY AND CONCLUSIONS
• LIST OF PARTICIPANTS