The importance of the management of waste has increased as the intensity of agricultural production on farms has increased. Waste from livestock production is dominated by manure, but also includes bedding and litter, wasted feed and water and soil. Table 1 lists some relevant characteristics of manure; human waste is included both for comparison and because it too must be treated on a farm. The high organic content of manure provides an excellent growth medium for bacteria. The metabolic activity of bacteria will consume oxygen and maintain bulk-stored manure in an anaerobic state. Anaerobic metabolic activity can produce a number of well-known toxic gaseous by-products, including carbon dioxide, methane, hydrogen sulphide and ammonia.
Table 1. Physical properties of manure as excreted per day per 1,000 lb of animal weight, excluding moisture.
|
Weight (lb) |
Volume (ft3) |
Volatiles (lb) |
Moisture (%) |
||
|
As excreted |
As stored |
||||
|
Dairy cow |
80–85 |
1.3 |
1.4–1.5 |
85–90 |
>98 |
|
Beef cow |
51–63 |
0.8–1.0 |
5.4–6.4 |
87–89 |
45–55 |
|
Pig (grower) |
63 |
1.0 |
5.4 |
90 |
91 |
|
Sow (gestation) |
27 |
0.44 |
2.1 |
91 |
97 |
|
Sow and piglets |
68 |
1.1 |
6.0 |
90 |
96 |
|
Laying hens |
60 |
0.93 |
10.8 |
75 |
50 |
|
Broilers |
80 |
1.3 |
15. |
75 |
24 |
|
Turkeys |
44 |
0.69 |
9.7 |
75 |
34 |
|
Lamb (sheep) |
40 |
0.63 |
8.3 |
75 |
— |
|
Human |
30 |
0.55 |
1.9 |
89 |
99.5 |
Source: USDA 1992.
Management Processes
The management of manure involves its collection, one or more transfer operations, storage or/and optional treatment and eventually utilization. The moisture content of manure as listed in table 1 determines its consistency. Wastes of different consistencies require different management techniques and therefore can present different health and safety hazards (USDA 1992). The reduced volume of solid or low-moisture manure generally permits lower equipment costs and energy requirements, but handling systems are not easily automated. The collection, transfer and any optional treatments of liquid waste are more easily automated and require less daily attention. Storage of manure becomes increasingly mandatory as the seasonal variability of the local crops increases; the storage method must be sized to meet the production rate and utilization schedule while preventing environmental damage, especially from water runoff. Options for utilization include use as plant nutrients, mulch, animal feed, bedding or a source to produce energy.
Manure Production
Dairy cows are typically raised on pastures, except when in holding areas for pre- and post-milking and during seasonal extremes. Water use for cleaning in milking operations can vary from 5 to 10 gallons per day per cow, where flushing of wastes is not practised, to 150 gallons per day per cow where it is. Therefore, the method used for cleaning has a strong influence on the method chosen for manure transport, storage and utilization. Because the management of beef cattle requires less water, beef manure is more often handled as a solid or semi-solid. Composting is a common storage and treatment method for such dry wastes. The local precipitation pattern also strongly influences the preferred waste management scheme. Excessively dry feedlots are apt to produce a downwind dust and odour problem.
The major problems for swine raised on traditional pastures are the control of runoff and soil erosion due to the gregarious nature of pigs. One alternative is the construction of semi-enclosed pig buildings with paved lots, which also facilitates the separation of solid and liquid wastes; solids require some manual transfer operations but liquids can be handled by gravity flow. Waste-handling systems for fully enclosed production buildings are designed to collect and store waste automatically in a largely liquid form. Livestock playing with their watering facilities can increase the volumes of swine waste. Manure storage is generally in anaerobic pits or lagoons.
Poultry facilities are generally divided into those for meat (turkeys and broilers) and egg (layers) production. The former are raised directly on prepared litter, which maintains the manure in a relatively dry state (25 to 35% moisture); the only transfer operation is mechanical removal, generally only once per year, and transport directly to the field. Layers are housed in stacked cages without litter; their manure can either be allowed to collect in deep stacks for infrequent mechanical removal or be automatically flushed or scraped in a liquid form much like swine manure.
The consistency of waste from most other animals, like sheep, goats and horses, is largely solid; the major exception is veal calves, because of their liquid diet. Waste from horses contains a high fraction of bedding and may contain internal parasites, which limits its utilization on pasture land. Waste from small animals, rodents and birds may contain disease organisms that can be transmitted to humans. However, studies have shown that faecal bacteria do not survive on forage (Bell, Wilson and Dew 1976).
Storage Hazards
Storage facilities for solid wastes must still control water runoff and leaching into surface and ground water. Thus, they should be paved pads or pits (that may be seasonal ponds) or covered enclosures.
Liquid and slurry storage is basically limited to ponds, lagoons, pits or tanks either below or above ground. Long-term storage is coincident with onsite treatment, usually by anaerobic digestion. Anaerobic digestion will reduce the volatile solids indicated in table 1, which also reduces odours emanating from eventual utilization. Unguarded below-surface holding facilities can lead to injuries or fatalities from accidental entry and falls (Knoblauch et al. 1996).
The transfer of liquid manure presents a highly variable hazard from mercaptans produced by anaerobic digestion. Mercaptans (sulphur-containing gases) have been shown to be major contributors to the odour of manure and are all quite toxic (Banwart and Brenner 1975). Perhaps the most dangerous of the effects from H2S shown in table 2 is its insidious capacity to paralyze the sense of smell in the 50- to 100-ppm range, removing the sensory capacity to detect higher, rapidly toxic levels. Liquid storage for as short as 1 week is enough to initiate the anaerobic production of toxic mercaptans. Major differences in long-term manure gas generation rates are thought to be due to uncontrolled variations in the chemical and physical differences within the stored manure, such as temperature, pH, ammonia and organic loading (Donham, Yeggy and Dauge 1985).
Table 2. Some important toxicologic benchmarks for hydrogen sulphide (H2S)
|
Physiological or regulatory benchmark |
Parts per million (ppm) |
|
Odour detection threshold (rotten-egg smell) |
.01–.1 |
|
Offensive odour |
3–5 |
|
TLV-TWA = recommended exposure limit |
10 |
|
TLV-STEL = recommended 15-minute exposure limit |
15 |
|
Olfactory paralysis (cannot be smelled) |
50–100 |
|
Bronchitis (dry cough) |
100–150 |
|
IDLH (pneumonitis and pulmonary oedema) |
100 |
|
Rapid respiratory arrest (death in 1–3 breaths) |
1,000–2,000 |
TLV-TWA = Threshold limit values–Time weighted average; STEL = Short-term exposure level; IDLH = Immediately dangerous to life and health.
The normally slow release of these gases during storage is greatly increased if the slurry is agitated to resuspend the sludge that accumulates at the bottom. H2S concentrations of 300 ppm have been reported (Panti and Clark 1991), and 1,500 ppm has been measured during the agitation of liquid manure. The rates of gas release during agitation are much too large to be controlled by ventilation. It is most important to realize that natural anaerobic digestion is uncontrolled and therefore highly variable. The frequency of serious and fatal over-exposures can be predicted statistically but not at any individual site or time. A survey of dairy farmers in Switzerland reported a frequency of about one manure gas accident per 1,000 person-years (Knoblauch et al. 1996). Safety precautions are necessary each time agitation is planned to avoid the unusually hazardous event. If the operator does not agitate, sludge will build up until it may have to be removed mechanically. Such sludge should be left to dry before someone physically enters an enclosed pit. There should be a written confined-space programme.
Rarely used alternatives to anaerobic ponds include an aerobic pond, a facultative pond (one using bacteria that can grow under both aerobic and anaerobic conditions), drying (dewatering), composting or an anaerobic digester for biogas (USDA 1992). Aerobic conditions can be created either by keeping the liquid depth no more than 60 to 150 cm or by mechanical aeration. Natural aeration takes more space; mechanical aeration is more costly, as are the circulating pumps of a facultative pond. Composting may be conducted in windrows (rows of manure which must be turned every 2 to 10 days), a static but aerated pile or a specially constructed vessel. The high nitrogen content of manure must be reduced by mixing a high carbon amendment that will support the thermophilic microbial growth necessary for composting to control odours and remove pathogens. Composting is an economical method of treating small carcasses, if local ordinances permit. See also the article “Waste disposal operations” elsewhere in this Encyclopaedia. If a rendering or disposal plant is not available, other options include incineration or burial. Their prompt treatment is important to control herd or flock disease. Swine and poultry wastes are particularly amenable to methane production, but this utilization technique is not widely adopted.
Thick crusts can form on top of liquid manure and appear solid. A worker may walk on this crust and break through and drown. Workers can also slip and fall into liquid manure and drown. It is important to keep rescue equipment near the liquid manure storage site and avoid working alone. Some manure gases, such as methane, are explosive, and “no smoking” signs should be posted in or around the manure storage building (Deere & Co. 1994).
Application Hazards
Transfer and utilization of dry manure can be by hand or with mechanical aids like a front-end loader, skid-steer loader and manure spreader, each of which presents a safety hazard. Manure is spread onto land as fertilizer. Manure spreaders are generally pulled behind a tractor and powered by a power-take-off (PTO) from the tractor. They are classified into one of four types: box-type with rear beaters, flail, V-tank with side discharge and closed tank. The first two are used to apply solid manure; the V-tank spreader is used to apply liquid, slurry or solid manure; and the closed tank spreader is used to apply liquid manure. The spreaders throw the manure over large areas either to the rear or sides. Hazards include the machinery, falling objects, dust and aerosols. Several safety procedures are listed in table 3.
Table 3. Some safety procedures related to manure spreaders
1. Only one person should operate the machine to avoid inadvertent activation by another person.
2. Keep workers clear of active power-take offs (PTOs), beaters, augers and expellers.
3. Maintain all guards and shields.
4. Keep persons clear of rear and sides of the spreader, which can project heavy objects mixed into the manure as far as 30 m.
5. Avoid dangerous unplugging operations by preventing spreader plugging:
- Keep stones, boards and other objects out of the spreader.
- In freezing weather, make sure flails and chains on flail-type spreaders are loose and unfrozen before operation.
- Keep chains and beaters on beater-type spreaders in good operating order by replacing stretched chains and avoiding dropping loads of frozen manure onto the spreader chains.
- Never get into an operating spreader to clean it.
- Maintain the unloading auger and discharge expeller on V-tank spreaders so they operate freely.
- In cold weather, clean the spreader insides so wet manure will not freeze the moving parts.
6. Use good tractor and PTO safety practices.
7. Make sure the relief valve on closed-tank spreaders is operative to avoid excessive pressures.
8. When unhooking the spreader from the tractor, make sure the jack that holds the weight of the spreader tongue is secure and locked to prevent the spreader from falling.
9. When the spreader is creating airborne dust or aerosols, use respiratory protection.
Source: Deere & Co. 1994.