Sewage Sludge as Soil Conditioner in Kuwait

4In this article we will discuss about the sewage sludge as soil conditioner in Kuwait.

Dr. Samir AI-Ghawas

Research Scientist Aridland Agriculture Department Kuwait Institute for Scientific Research P.O. Box 24885, 13109 Safat, Kuwait

Introduction:

The use of soil surface layer as a disposal medium is potentially an efficient, environmentally sound and economically accepted solution to the problem of waste disposal. The objection of land treatment is to have the diverse and naturally occurring soil organisms to use the large surface area provided by the soil system to assimilate, decompose and biodegradate the waste component through a series of complex reactions to yield carbon dioxide, water and immobilized constituents.

Although municipal sewage and products of its treatment are successfully treated by land spreading practices, the emphasis have been on disposal of society’s unwanted wastes rather than its utilization as an effective mean to enhance soil-water-plant relationships and built-up soil resources.

Urban waste contain mostly organic substances which are residues or derivatives of human, vegetable and animal materials, which were originally derived directly or indi­rectly from the soil. These residues therefore, should not be allowed to remain merely as wastes, but should be recovered and suitably treated in order to benefit from the fertilizing and soil conditioning values.

With increasing population density and industrial expansion, the need for treatment and disposal of wastewater has grown. Increasing human water requirement and demands for cleaner environment make sewage treatment essential. However, as quantities of sewage increased and in-plant treatments were perfected, land application of sewage sludges became a means of disposal rather than a method of salvaging plant nutrients.

If some way can be found to safely recycle this sludge onto our land, it will assist in solving two of the most demanding current problems:

1. To find an environmentally safe way to dispose of wastewater treatment by-­products, and;

2. To find a way to enhance the physical and chemical properties of our extremely degraded soil resources.

The goals of applying sewage sludge to land should be achieved in a way that the land is not impaired, regardless of whether the emphasis is on discarding or utilizing this product.

In this paper, the effect of sewage sludge and the product of its biodegradation on changes to some soil physical and chemical properties will be discussed and qualitatively assessed. However, to appreciate the sludge conditioning value to Kuwaiti soils, one need to examine the current status of our land, especially its mineral composition and organic content. This will expose the extreme level of degradation that our soils are under and how much they are in need to build their organic constituents.

Local Soil Properties:

The soils of Kuwait have general features common to all of them, these include; sandy texture, very low organic matter content, presence of calcite and levels of salinity that in some places are extremely high. Some physical and chemical soil properties from Kuwait Institute for Scientific Research Station in Sulaibiya are presented in Tables 1 and 2 respectively.

It is clear from Table 1 that the soils are predominantly sandy with more than half in the 200-600 µm medium sand range. This would have great influence on soil water behaviour especially excessive infiltration and low water holding capacity.

Soil calcite fraction ranged from 11 to 28 per cent equivalent which indicate that the soils of the area are moderate to highly calcareous. While gypsum is present in low amounts (<1%) in non-irrigated soils. Ground water are brackish with high content of dissolved SO₄^2- which if it is the sole source of irrigation water would lead with time to a build-up of soil gypsum content.

Soil chemical analysis is presented in Table 2 and its values confirm the calcareous nature of the soil by buffering the soil reaction to a pH of around 8.0. Soil salinity is low with levels of around ECe 1.0 dSm^-1. Soil saturation extract is dominated by sulfate followed by chloride with base cations present with the following declining order, Ca²⁺>K⁺>Na⁺>Mg²⁺.

Soluble sodium is low and therefore the calculated sodium adsorption ratio (SAR) reflect lack of soil sodicity problems. When considering however, the fertility or plant nutrients availability the soil is extremely poor. Organic matter content is very low and is restricted to the surface layer. Levels are estimated at about 0.003 per cent, however, this is within the limits of accuracy in determining soil organic matter.

Nitrogen levels, therefore are extremely deficient since most of soil nitrogen resides in the organic fraction. Plant available phosphate are also extremely low, only where heavy fertilization with farm yard manure and inorganic fertilizers were practiced for over 10 years did available -P become adequate. Only potassium out of the major macro-nutrients was present in sufficient supply.

Metal ions extracted by the DTPA plant availability test indicated that this soil contains low levels of iron and zinc, while those of manganese were adequately supplied.

Sludge Nutritional Benefits:

The beneficial effects of sewage sludge on crop productivity have been widely demonstrated. Crop response is usually related to the nitrogen and phosphorus supplying capacity of the sludge. Each ton of dry sludge is estimated to contain about 50 kg of nitrogen, 30 kg of phosphorus and 5 kg of potassium.

Table 3 shows the average elemental composition of sewage sludge in USA. However, comparing crop yield from sludge- amended soils, with those receiving equivalent nutritional amounts in inorganic commer­cial fertilizer form indicated yields were always higher with the sludge amendments. This was attributed to the improvement in the soil physical properties.

Decomposition of Sewage Sludge in the Soil:

The introduction of sludge into the soil system promotes the development of chemi­cal reactions (redox reactions, hydrolysis, etc.), physicochemical reactions (dissolution, precipitation, adsorption, bounding, etc.), and micro-biological processes (microbial mul­tiplication, metabolic production, enzymatic reactions, etc.). Organic substances released in the medium, regardless of their origin, are involved in various interactions with the mineral fraction of the soil. The product of these interactions and their evolution with time is essential for the modification of the soil physical properties.

This degradation of sludge or its products when applied to soils would be governed to a great extent by its composition. Micro-organisms, mediating this decomposition process would have a cellular carbon/nitrogen ratios of approximately 10, and would use the C and N released to build new living tissues.

Therefore, during microbial degradation of a waste like sludge characterized with a high C: N ratio, soil nitrogen would be immobilized and if it is in short supply rate of degradation would be retarded. While on the other hand, if the waste incorporated had a low C: N ratio then degradation would be rapid and the excess nitrogen would be released into the soil environment.

Furthermore, since the microbial degradation of the sludge is controlled by enzymatic activities, then a favourable temperature and moisture regimes would enhance the rates of decomposition. Maximum rate of biological degradation of sludge products would take place at 30 – 35°C, however, the varied and rich composition of soil microbial population will still degrade the waste at other less favourable temperatures (Fig. 1).

Soil moisture affects the decomposition of sewage sludge in two ways. If moisture is low, then water needs of soil micro-organisms is insufficient. If moisture is too high then oxygen supplies to the micro-organisms becomes restricted. In between, there is an optimum moisture content for microbial activities, this usually corresponds to soil mois­ture contents of field capacity (-0.25 – 1 bar water suction potential). Close to this optimum, there is little effect of moisture changes on rate of decomposition.

The effects of sludge soil loading rate and the methods of its application on the degradation rate is not well investigated or understood. Furthermore, many reports indicate mixed results. It could be the indirect affect on soil aeration that dictates the rate of decomposition.

Effect of Sludge on Some Soil Physical Properties:

The persistence of the effect of sludge on soil physical properties is directly related to the persistence of sludge organic carbon in the soil medium. If part of the sludge organic carbon is resistant to degradation, then the soil organic carbon content will increase, and under repeated sludge application and time, an irreversible alteration of the physical properties will occur.

Three major soil properties will be discussed below:

A. Water Retention:

Sludge soil application would affect the water retention in two ways. The first is directly related to the hydrophilic behaviours of the sludge organic particles, which are characterized with high water adsorption capacity and thus higher soil-water retention.

Increases in the water retention capacity at field capacity (Matric Potential (MP) = -0.33 bar) and at wilting point (MP = -15 bar) as a consequence of sludge application are widely reported. However, changes to the available water content (AW) are still a subject of controversy.

Water available for plant growth is measured by difference in moisture contents at (-0.33 bar) and at (-15 bar). Since water retention (on a weight basis) will increase at both field capacity and wilting points when organic wastes are added, then AW may stay unchanged.

Similarly, Khaleel (1981) has indicated that even when increases in AW were evident on a weight basis, they were not significant on a volumetric basis. Moreover, the relative increases in water retention capacity were greater for coarse-textured than for fine-textured soils (Fig. 2). It was found that the AW of a sandy loam soil has increased by 35 per cent after 9 years of sludge application. This support the suggestion, that organic matter addition might increase AW only in very sandy soils, with low intrinsic AW.

The second way that sludge will affect soil-water retention is by an indirect modifi­cation to the soil other physical properties, such as bulk density, porosity, and pore size distribution. Water held inside the soil at wilting point is surface adsorbed, so the increase in water retention at (-15 bar MP) is explained by an increase in surface area due to the incorporation of organic waste.

While at lower tension approximat­ing field capacity, the increases in water retention is partly attributed to an increase in the number of microprobes.

B. Soil Structure:

Soil structure is defined by the distribution and arrangements of soil particles into recognized and uniformed aggregates. Organic carbon derived from sludge application, was the main factor responsible for water stability of soil aggregates and an increase in their number and size.

Scanning electron microscopy (SEM) examination of water-stable aggregates (WSA) revealed larger size and a looser and more porous internal structure for sludge amended soils. Furthermore, in a study by Epstein (1975), showed percent stable aggregates increased as a result of sludge addition.

After 175 days, the per cent (WSA) averaged 28-35 per cent for the sludge amended soils, as compared to 17 per cent for the untreated soil. Potential binding agents between soil minerals particles may include filamentous microbes (fungus hyphae), metabolites produced by decomposers as well as organic substances introduced by the sludge products.

However, closer exami­nation indicated that fungal activity was the main contributor to the increases in the WSA contents following sludge application (Fig. 3). This may take the form of segmentation of primary particles by fungal polysaccharides and by the physical entanglement of primary particles in the web of fungal hyphae.

C. Hydraulic Conductivity and Infiltration:

Information on the effect of organic waste such as sludge on soil hydraulic conduc­tivity or water transmission is limited with high variability in actual measurements. However, for soils, with wide range of textures, the saturated hydraulic conductivity constant (K_sat) was usually higher for plots amended with sludge others have obtained increases in both K_sat and K_unsat, but only at large scale sludge application (Fig. 4).

There are few reports on decrease in K_sat following sludge application, but this was for a short period. It could be that initial sludge decomposition products, microbial metabolites or fungal mycelia, may have led to plugging of soil pores especially if it is of a fine texture. Others have attributed the reduction on the coarse textured sandy soils to the hydrophobic nature of some materials in the sludge.

Water infiltration would depend on the hydraulic conductivity of the underlying soil, and therefore the beneficial effect of sludge on water stability of soil aggregates explain why soils can be protected against crust formation which would impede water infiltration.

Potential Hazards:

To this point the beneficial effects of sludge land application on both the fertilizing and soil conditioning aspects have been stressed. However, before sludge can be widely used in land application and agriculture production, some questions about its adverse effects must be answered.

The most frequent asked questions are:

1. Would the heavy metals contained in the sludge, contribute to a build-up of heavy metals in the soil and thus pose one of the greatest potential hazards to land, crop and the food and feed chain?

2. Would disease pathogens, some of which would survive sewage treatment and reach the soil during land application pose a threat to the general health?

3 What long term detrimental affect may be associated with sewage sludge land application and would that cause degradation to soil resources?

These questions have been addressed by other and their general recommendations and guidelines should be adhered to. This would minimize the negative impacts of sludge application to crop land and the health hazards associated with such practice, and this would allow the recovery of the nutritional and conditional properties of this products.