In the previous article the handling of manure, the various kinds of floors and channels as well as the flush system were discussed. In this article the structures and equipment for the handling of manure will be discussed. The series was compiled by the ARC’s Institute for Agricultural Engineering.First there should be looked at the disposal of disease-infected carcasses. This carcass must be disposed of by means of burning of incineration. If none of these are possible, the relevant carcasses must be disposed of by burying them at least 1,8 m deep – in caustic lime is possible – especially in case of large animals and rabid animals, or animals thought to have been infected by rabies. The carcasses of small animals or the remains of burnt or incinerated carcasses must be buried at least 1,2 m deep.
The purpose of a biologically active anaerobic lagoon is to serve as an odourless long-term reservoir for liquid manure. A lagoon system allows for high flexibility regarding timing for application to fields to relieve moisture stress in crops. The need for plant nutrients can be satisfied partially, depending on the time of application.
Biologically active lagoons work better during summer than in winter. If the water temperature of a lagoon falls below 10 to 13ºC, the anaerobic conversion of organic material into methane gas and carbon dioxide will not take place on a practical scale. Problems experienced with improperly designed and managed lagoons include bad odours, rapid slurry and the build-up of heavy metals in the lagoon sludge.
Single or double lagoons
A second lagoon, smaller than the primary anaerobic lagoon, is generally recommended for intensive pig units which recirculate the lagoon liquid for channel flushing where pigs have direct access to flushing water. It is believed that the second lagoon will provide a certain degree of protection against disease-carrying organisms carried over from the first lagoon.
The second lagoon has the advantages of that when the primary lagoon is desludged from time to time, liquid manure inflow may continue into the second lagoon, and lagoon liquid from the secondary lagoon may be used to irrigate pastures, since toxic copper will concentrate in the slurry of the primary lagoon.
Design considerations for anaerobic lagoons
The location of the lagoon should be adjacent to the piggery, or as close to it as is practically possible. Lagoons have to be at least 300 m from residential areas at a place where prevailing wind will carry bad odours away from houses. Lagoons should be located in soils of low permeability which seal quickly.
Normal liquid manure and flushing water storage capacity should be enough for at least 90 days. In dry regions with high evaporation rates, the retension time may be longer than 200 days. A lagoon has to be at least 2,0 m deep. Anaerobic lagoons function well over a wide range of input loads up to a given lagoon volume. The criteria for the size of the lagoon are not compatible. If odor control is the main aim, the lagoon volume per unit of body weight should be as large as possible.
Cellulose matter, such as straw, sawdust or woodchips, are not degraded by anaerobic bacteria as rapidly as fat, proteins or soluble carbohydrates. Bedding material and undigested cellulose may result in rapid sludge build-up, especially in overloaded lagoons. The removing of bedding and undigested cellulose from the lagoon inflow by means of screening or sedimentation is essential.
The minimum top width should be 1,5 m. The side slope on the dry side of the embankment should not be steeper than 2:1. The inlet and outlet of double lagoon systems should be located at opposite sides to prevent short-circuiting and under-utilisation of lagoon volumes by active bacteria.
A large lagoon surface increases the evaporation rate and consequently also the slat content. When lagoon water is recycled to flush out the manure, the salt accumulates faster. A moderate increase in salinity stimulates biological activity.
Removal of sludge
Sludge has to be removed when the displacement effect of the slurry has reduced the liquid volume to below required level for a stable bacterial population. Sludge build-up which takes up about half of the total lagoon volume, is possibly the maximum level allowed before bad odours occur.
Manure storage tanks and agitators
An alternative method of storing manure is the use of a concrete manure tank. Depending on the specific situation, a single or double manure tank system or a manure tank together with a lagoon may be used. In any case, the manure tank will serve as a reservoir for liquid manure coming from the pig pens. The volume of the tank is calculated according to the duration the liquid manure is to be stored – usually no longer than seven days.
The function of the agitator/s is to bring the water and solid parts into suspension before these are being pumped. It is essential to prevent the manure tank from silting up and/or the pump from not operating effectively.
Structures and equipment for separating solid manure
For practical reasons it is strongly recommended that bedding and undigested cellulose are removed from pig excreta prior to storing liquid manure in a lagoon, or prior to applying it to farm-lands by sprinkler. Non-mechanical structures such as small settling ponds, or barriers across a natural depression made of railway sleepers or straight wooden beams or bales of hay, can be used effectively and require almost no maintenance. Mechanical equipment that does not require much supervision and maintenance includes the static horizontal.