Sewage sludge and organic composts

Written and compiled by:

Ahmad Mahmood, Department of Agronomy, Pir Mehr Ali Shah-Arid Agriculture University, Rawalpindi

Correspondence: [email protected],  


Sewage is water-carried waste, in solution or suspension that is intended to be removed from a community. Also known as waste water, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains.


Sludge refers to the residual, semi-solid material left from industrial waste water or sewagetreatment processes. It is a mixture of water and solids separated from various types of water as a result of natural or artificial processes.

Sewage sludge: sludge from urban waste water treatment plants, whereby ‘urban waste water’ is understood as: “domestic waste water or the mixture of domestic waste water with industrial waste water and/or run-off rain water”.

Classes of sewage include





v.Surface Runoff 

The wastewater from residences and institutions, carrying body wastes, washing water, food preparation wastes, laundry wastes, and other waste products of normal living, are classed as domestic or sanitary sewage.

Liquid-carried wastes from stores and service establishments serving the immediate community, termed commercial wastes (similar to household flows).

Wastes that result from an industrial process or the production or manufacture of goods are classed as industrial wastewater. Their flows and strengths are usually more varied, intense, and concentrated than those of sanitary sewage.

Surface runoff, also known as storm flow or overland flow, is that portion of precipitation that runs rapidly over the ground surface to a defined channel. Precipitation absorbs gases and particulates from the atmosphere, dissolves and leaches materials from vegetation and soil, suspends matter from the land, washes spills and debris from urban streets and highways, and carries all these pollutants as wastes in its flow to a collection point.

Strict regulations on discharge of sewage into waters, higher costs of alternate disposal procedures, and higher prices of chemical fertilizers have increased interest in the use of sewage sludge in crop production.


Land application of sewage sludge can have many beneficial effects.

  • ·Supplying nutrients (N, P, secondary nutrients, and micronutrients) to the crops
  • ·improving soil physical properties
  • ·increasing soil organic matter content
  • ·heavy metals reducing micro-organisms


Although these are obvious benefits, there are also concerns that must be addressed to ensure a safe, economical, and environmentally sound approach to applying sewage sludge to the soil. The most commonly voiced concerns include:

1. The potential for nitrate or phosphate contamination of waters

2. The potential for damage to soils, plants, animals, and humans because of possible toxic metal applications

3. The potential for pathogen transfer.


Application of sewage sludge to cropland can result in

a.Soil contamination


c.Accumulation of trace elements in the food supply.

The magnitude of the problem depends on the interrelationships of a number of factors, such as the

a.Composition of sludge

b.The rate and frequency of applications

c.Soil characteristics

d.Plant species.


Trace elements have been defined as the elements that occur in natural systems in small amounts and that, when present in excessive concentrations, are toxic to living organisms.


Important trace elements are Cadmium, Selenium, and Molybdenum which can cause toxicities in animals and humans. Molybdenum and Selenium are present in sewage sludges in low concentrations, thus these elements normally do not limit the rate of sludge application to soil. Cadmium, however, has caused adverse health effects in humans who ingested plants grown on soils contaminated with high levels of Cadmium.

Ingestion of soil or a sludge-soil mixture by grazing animals provides a direct pathway to adding excessive trace elements (especially Lead) to the food chain.


The Council for Agricultural Science and Technology (CAST, 1976) classified Cadmium, Copper, Molybdenum, Nickel, and Zinc as potential hazards in land application of municipal sewage sludges. These metals tend to accumulate in plants and cause either reduced yields or health problems to animals or humans that ingest the plants. In Pakistan, there is still no regulation regarding application of sewage sludge.

Sludge Composition:

Trace element concentrations in sludges exhibit wide variations from one city to another and from season to season within a single treatment plant. The extent and nature of the industrialization in the sanitary district and the pretreatment requirements largely determine the metals that will be present in municipal sewage sludge. Low levels of trace elements in domestic sewage sludge are present from food and human wastes, from plumbing and wastewater transport systems, and from surface runoff.


Metals exist in different forms in municipal sewage sludges. Their form depends on the chemical properties of the sludge and on the chemistry of the metal. Different forms of Cu, Zn, Pb, Cd, and Ni present in municipal sewage sludge as follows:

Cu: sulfides > carbonates > organic bound= adsorbed > exchangeable

Zn: organic bound > carbonates > sulfides> adsorbed > exchangeable

Pb: carbonates > organic bound > sulfides> adsorbed > exchangeable

Ni: carbonates > organic bound > exchangeable> adsorbed > sulfides

Cd: carbonates > sulfides > organic bound> adsorbed = exchangeable


Based on this data, less than 17 percent of the total amount of Cu, Zn, Pb, and Cd in sludges and approximately 22 percent of Ni are in the sorbed and exchangeable fractions, that is, the forms readily available to plants.

The remainder of the metals is present in forms which require conversion to water-soluble, exchangeable, or sorbed forms before uptake by plants.



Composting is a technique used to speed up the natural decay process. The technique converts organic wastes to a mulch which is used to fertilize and condition soil. Leaf waste decomposes naturally in about two years. Composting can take as long as a year or as little as 14 days, depending upon the amount of human control.

Compostable Materials:

Most farm wastes can be composted, including leaves, grass clippings, plant stalks, vines, weeds, twigs and branches. Compostable food wastes include fruit and vegetable scraps, coffee grounds, eggshells and nutshells. Other compostable materials are hair clippings, feathers, straw, livestock manure, bone meal and blood meal.

Materials should NOT be composted if they promote disease, cause odors, attract pests, or create other nuisances. These include meat, fish, poultry, dairy products, foods containing animal fats, human/pet feces, weeds with developed seed heads, and plants infected with or highly susceptible to disease, such as roses and peonies.

Materials that should be composted only in limited amounts include wood ashes (a source of lime), sawdust (requires extra nitrogen), plants treated with herbicides or pesticides (the chemicals need time for thorough decomposition), and black and white newsprint (composts slowly, so it should comprise no more than 10% by weight of the total pile).

Composting Requirements:

  1. Shredded Organic Wastes: Shredding, chopping or even bruising organic materials hastens decay. One way to shred leaves is to mow the lawn before raking, collecting the shredded leaves in the mower bag. It takes at least 34 cubic feet of shredded material to form a compost pile.
  2. Good Location: The compost pile should be located in a warm area and protected from overexposure to wind and too much direct sunlight. While heat and air facilitate composting, overexposure dries the materials. The location should not offend neighbors.
  3. Nitrogen: Nitrogen accelerates composting. Good sources include fresh grass clippings, manure, blood meal and nitrogenous fertilizer. Lime should be used sparingly if at all. It enhances decomposition, but too much causes nitrogen loss, and it usually isn`t necessary unless the pile contains large amounts of pine and spruce needles or fruit wastes.
  4. Air: The compost pile and its enclosure should be well ventilated. Some decay will occur without oxygen, but the process is slow and causes odors.
  5. Water: Materials in the compost pile should be kept as moist as a squeezed sponge. Too little or too much water retards decomposition. Overwatering causes odors and loss of nutrients.

Making Compost:

Materials for making compost:

All organic materials, e.g. plant wastes or residues can be used in making compost. The materials include dry and wet materials.

Dry Material: Little water content, with high carbon/nitrogen (C/N) ratio which decomposes slowly. e.g. Rice straw, sugarcane residue residues, banana leaves, rice husk, tree leaves.

Wet Materials: High water content, with low C/N ratio which decomposes quickly. e.g. Chicken manure, animal manure, green manures, leaves of leguminous trees, grass clippings.

The mixture used in compost usually is: 6 Dry materials: 3 wet materials: 1 soil

Carbon – to – Nitrogen Rates:

All living organisms are made of large amounts of carbon (C) combined with small amounts of nitrogen (N). The balance of these elements in an organism is called the C/N ratio. This C : N ratio is an important factor determining how easily bacteria can decompose organic waste during the composting process.

The microorganisms in compost use carbon (C) for energy and nitrogen (N) for protein synthesis. The proportions of these two elements used by the bacteria averages about 30 parts of carbon to 1 part of nitrogen. Given a steady diet of C : N at 30:1 ratio, bacteria can work on organic material very quickly.

Brown Materials (dry – high carbon wastes):

Hay, leaves, corncobs and cornstalks, nutshells, sawdust, straw, vegetable stalks

Green Materials (wet – higher nitrogen wastes):

Coffee grounds, vegetables scraps, cover crops (green manure), fruit wastes and grains, grass clippings, manure, seaweed, weeds, eggs and eggshells, feathers, fish and seafood scraps, most animal scraps slow down decomposition

Procedure of Making Compost:

A. Piling up compost heap:

1. Make a movable wooden frame. This serves as a guide in stacking, as the piles go higher.

2. Select a level area near a water source, and it also receives an equal amount of sunlight and shade.

3. Place a layer of dry brown materials such as branches, leaves and straw at the base of the pile first. These are high carbon materials, provide for air circulation from the bottom of the pile.

4. Place green wet materials such as grass clippings or manure (high nitrogen materials) as the second layer.

5. Add a thin third layer of soil. This forms the first pile.

The purpose of using soil is to introduce microorganisms needed to break down the organic matter in the pile.

6. The ratio of the materials for piling the heap is:

Dry materials: Wet materials: Soil = 6: 3: 1

7. Place a hollow bamboo stick in the center of the pile for aeration.

8. If there is a lot of sappy materials in the heap, no additional water is needed. If the material is dry, it should be watered well as the heap is built.

9. Repeat the procedures 3 to 5, move the wooden frame up as the additional piles are made. Build the heap until 4 feet high.

The purpose of having the heap up to 4 feet high is:

• To develop and retain heat to destroy harmful bacteria.

• To generate fungus and good bacteria. The fungus produces antibiotics which destroy most harmful bacteria and break down the plant structure rapidly.

• The good bacteria and microorganisms begin to thrive and take over as the compost heap cools.

10. Remove the wooden frame.

11. Cover the heap surface to prevent excess rain from soaking or sun drying up the heap.

12. The temperature should reach 55° to 70 °C, after a few days and it will kill off most pathogen, weeds and seeds.

B. Turning Compost heap:

1. Turn the pile every 2 weeks.

The purpose of turning is to improve aeration, speed up micro-organism activity and ensure uniform decomposition. The pile is reconstructed, material previously on the top and sides of the pile should be moved to the center.

2. During the decomposition process, if there is not enough moisture, water the pile to maintain conditions conducive for the composting process.

3. If the heap is too wet and too compacted, turn it and sprinkle dry soil or dry materials on it.

4. The decomposing process usually takes 3 to 4 months, it depends on the air temperature and the mixtures.

5. The compost is finished decomposing when the pile cools off, and the volume decreases to about one-third of its original volume.

6. Good compost smells good, and is black brown in colour, crumbly and has an earthy odour with pH 7.0 to 7.2.

7. It is not necessary to add limestone to the compost pile, as the organism function well with a pH between 4.2 and 7.2. The compost naturally becomes less acid as it matures.

8. The length of time necessary for the composting process depends on several conditions.

• Carbon – to – nitrogen ratio of the materials used

• Surface area of particles

• Aeration

• Moisture

• Temperature

Advantages of Using Compost in Crop Cultivation:

• Improve soil structure

• Increase soil fertility

• Improve water / moisture retention

• Increase resistance of plants to pests and diseases

• Provide nutrients to plants

Diagnosing Composting Problems:

1. The pile is producing a bad odour

The pile may be too wet, too tight, or both. Turn it to loose and allow better air exchange in the pile.

If it is too wet, also turn the pile, but at the same time, add dry new materials.

Odour may indicate that animal products are in the compost pile.

2. No decomposition seems to be taking place.

The pile is too dry. Moisten the materials while turning the pile.

3. If the compost is moist enough and the centre is warm but not hot enough for complete breakdown.

The pile is too small. Collect more materials to make a larger pile. Turn and mix the old ingredients that may have only slightly decomposed into the new pile.

If the pile is not small, more nitrogen may be needed.

4. If the pile is moist, sweet smelling, with some decomposition, but still does not heat enough.

There is not enough nitrogen available for proper decomposition. Mix a nitrogen source such as fresh grass clippings, manure or fertilizer into the pile.


Building an enclosure:

Enclosing the compost pile saves space and prevents litter. The enclosure should be collapsible or provide an entry large enough to permit the pile to be turned. It should measure at least 4′ x 4′ x 4′ (a pile under 3 cubic feet generally does not decompose properly), but no taller than 6′ (too much weight causes compaction and loss of oxygen). The enclosure can be built of wood, pallets, hay bales, cinder blocks, stakes and chicken wire, or snow fencing. Prefabricated compost bins are also available.

Building the Pile:

Aside from the basic requirements for decomposition and preventing odors and other nuisances, there is no set method for building a compost pile. One technique may be faster than another, but a variety of methods work well. Piles can be built in layers to ensure the proper proportion of carbon (e.g., leaves, woody materials) to nitrogen (grass, fertilizer), but the layers should be thoroughly intermixed after the pile is built.


Turning and mixing the pile with a pitchfork or shovel, or shifting it into another bin, provides the oxygen necessary for decomposition and compensates for excess moisture. A pile that is not mixed may take 34 times longer to decompose. Recommendations for mixing the pile vary from every 3 days to every 6 weeks. More frequent turning results in faster composting. Odors indicate that the pile is too damp or lacks oxygen, and that more frequent turning is necessary.

Occasional watering may be necessary to keep the pile damp, especially in dry weather. Covering the pile with black plastic reduces the need for watering; it also prevents rainwater from leaching out the nutrients.

A pile that is decomposing properly should generate temperatures of 140°-160°F at its center. The heat kills most weed seeds, insect eggs and diseases. The pile should be turned when the center begins to cool. Turning the pile maintains the temperature and ensures that all material is exposed to the center heat. When the compost is finished, the pile will no longer heat up.

Small amounts of fresh materials may be added but should be buried inside the pile to avoid pests and speed composting. It is better to add fresh materials to a new pile.

Finished Compost:

Finished compost is dark brown, crumbly, and has an earthy odor. Depending upon seasonal temperatures, a well-built, well-tended pile generally yields finished compost in 2 weeks to 4 months. An unattended pile made with unshredded material may take longer than a year to decompose.

Sample Instructions for Fast Composting:

  • shredded leaves (about 2/3 by volume)
  • fresh grass clippings (about 1/3 by volume, or slightly more for faster decomposition)
  • kitchen scraps (grind in blender)

Begin the pile with a 4″ layer of leaves. Add a 2″ layer of grass clippings. Repeat the layers until the pile is about 4′ high, and then add the kitchen scraps.

Chop vertically through the pile with the tines of a pitchfork to thoroughly bruise and mix the materials. Add just enough water to moisten the pile, and then cover it with a black plastic garbage bag. Using the same chopping technique, turn the pile on the second day after the pile is built, again on the fourth day, then every three days until the compost is finished. Except in dry weather, no further watering should be necessary.

The compost should be finished in about two weeks.

Alternate Composting Methods:

Compost can be made in a garbage can, barrel or drum that has a secure lid. Drill holes in the sides and bottom of the container to allow for air circulation and water drainage, and place it upright on blocks. Fill 3/4 of the container with organic wastes, add a little nitrogenous fertilizer (about 1/4 cup for a 55gallon barrel), and moisten the materials. Every few days shake the container or turn it on its side and roll it to mix the compost. The lid should be removed after turning to allow air penetration. This method yields finished compost in about 24 months.

Another method is to use a 30 or 40gallon plastic garbage bag. Fill the bag with organic materials, nitrogen and lime (one cup per bag helps counteract acidity caused by anaerobic composting). Shake well to mix materials. Add about 1 quart of water and close the bag tightly. Bags can be stored outdoors in the summer and in a heated basement or garage during the winter. No turning or additional water is necessary. The compost should be finished in about 6 12 months.


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Muhammad Ramzan Rafique
Muhammad Ramzan Rafique

I am from a small town Chichawatni, Sahiwal, Punjab , Pakistan, studied from University of Agriculture Faisalabad, on my mission to explore world I am in Denmark these days..

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