Solids-Liquid
Separation of Manure
Introduction
Liquid manure typically has a total solids
concentration of 3 to 6%. However, some farmers are handling liquid manure
with over 10% solids. This range in moisture con-tent is due to such factors
as method of storage, type of livestock, feeding program, and type and amount
of bed-ding used (if any).
The ratio of solids to liquids can be reduced simply by adding more dilution
liquid. However, this increases the volume of material that must be handled.
Also, it does nothing to reduce the size of particles that must be han-dled.
There are several advantages to reducing the amount of solids in the manure
and at the same time reducing the particle size of the solids:
Less possibility of plugging transfer pipes - The larger solid particles in
manure tend to build up on rough sur-faces inside a piping system. Sometimes
this results in a plug being formed. The effluent from a solid/liquid separator
has far less chance of this happening. Less power needed to pump the same
volume of material - Pumps are rated on their ability to pump water. As the
percentage of solids increases, so does the viscosity (resistance to flow).
This makes the manure harder to pump. For the same volume of manure, more
power is needed as the viscosity increases. Much easier to irrigate - Based
on the previous two reasons, it is obvious that when manure must be pumped
through pipe for several hundred metres, the lower the percentage of solids
in the manure, the easier it is to pump. Also, there is less chance of plugging
the pipes. Problems can arise when irrigating manure with high solids content
due to high pressures in the pipe near the pump. The greater the percentage
of solids in the manure, the higher this pressure must be in order to achieve
the desired pressure at the irrigation gun. These high pressures can result
in ruptured seals, leading to manure spills. Agitation time reduced - With
separated liquid, very little agitation is needed prior to removal from storage.
There is some settling of suspended solids, but these par-ticles are very
small and are usually easy to stir up. Other uses for solids - Especially
with the mechanical separators, the solids are relatively easy to store and
can be handled with conventional solid manure equipment. Sometimes, these
solids have other uses. Some farmers recycle the solids as bedding material.
Others re-feed some of the solids to livestock, while others compost and sell
the solids. Other uses for liquids - The separated liquids are ideal for in-barn
flushing systems. Gutters may be flushed regularly using this liquid. It has
a greater ability to remove solids than does unseparated manure. Odor control
- Farmers with separators are finding that the total level of odor from the
solid and liquid manure storages is less than the level of odor would have
been with no separation. If the liquid is being treated for odor control (either
chemically or by aeration, the job is much easier if the solids content is
low.
Along with these advantages of separation, there are several disadvantages:
The cost is relatively high when considered on its own. Some mechanical separators
cost around $15,000. To some farmers, this would be one of the most expensive
parts of the manure system and it may be hard to justify the purchase. The
mechanical systems are working in a material that is hard on machines. Regular
maintenance is needed to avoid breakdowns. Extra space is needed to accommodate
the system. Some of the mechanical units are compact and require very little
extra space. On the other hand, the settling basins occupy about 20% of the
area of the long-term storage. Two separate manure handling systems are needed
-one for liquid and one for solids. While a farmer may have spreading equipment
for the liquids, he may have to hire someone to spread the solids or purchase
his own spreading equipment. Some systems have high energy costs to operate.
The different systems have widely varying energy needs. Some may require no
energy inputs while others may require several kilowatts of power. There is
an increased management requirement. An extra step has been added to the manure
handling system. Someone has to take time to make sure the system is operating
properly. Again, this will vary depending on the type of separator. The separation
of solids from liquid is usually, achieved in one of two ways: (a) by using
the effects of gravity, or (b) by using a mechanical device.
Gravity Separation
Gravity separation, or sedimentation, is a
fairly simple means of removing solids from manure. It involves a settling
pond or basin. The flow of the liquid is slowed down to a point where solid
particles settle. Larger, heavier solids settle first; floating or suspended
solids won't settle. The liquid portion usually spills over or is pumped to
a separate storage. A fairly high percentage of solids can be removed from
the manure using this system. The settled portion has a high moisture content
and is handled as a thick slurry. This system of separation is not common
in Ontario but is used extensively in certain areas of the U.S.A. The most
common application is with feedlots where large amounts of runoff have to
be handled. The system can be used with other types of liquid manure, however.
Figure 1 shows a settling basin which is part of a swine manure handling system.
Figure
1. Liquid/solid separation using sedimentation in 2-stage storage.
Mechanical
Separation
Separation of solids and liquids by mechanical
means has some advantages over the sedimentation method:
The separated solids portion has a lower moisture content and can usually
be handled as solid manure. Most systems are better equipped to remove large
floating or suspended particles. Less space is needed. There are basically
two methods of mechanical separa-tion: (a) size separation by screening; and
(b) size separa-tion by centrifugation. Because each of these has its own
advantages and disadvantages, we often see equipment which incorporates the
best of both systems. The perfor-mance of these units is governed by many
factors and, as such, it is hard to generalize about the capabilities of mechanical
separators. However, it is reasonable to expect the amount of solids removed
to vary from about 40 to 80% . The moisture content in the separated solids
portion will likely be between 70 and 90%. Through put rate will vary greatly,
but on most farms will likely be in the range of 3 to 12 cubic metres per
hour (approx. 700 to 3000 gal./hr.).
Following is an individual discussion of several systems available in Ontario.
Stationary Rundown Screen: Liquid manure flows onto an inclined screen. The
fluids run through the holes, leav-ing the solids trapped on the surface.
As the solids ac-cumulate, they move downward due to fluid pressure and gravity
forces. The solids fall off the end of the screen and are handled as solid
manure. One of the problems with the screen is that a biological slime builds
up and plugs the openings. Frequent brushing is needed to keep the holes unplugged.
The stationary rundown screen is the least ex-pensive mechanical separator.
Figure
2. Stationary run-down screen.
Vibrating Screen: This has a similar
function to the sta-tionary screen. The difference is that a short, rapid
reciprocating motion is employed to move the solids to the edge of the screen.
The power requirement is higher with this system. Like the stationary screen,
this system needs frequent cleaning to remove the slime build up.
Figure
3. Vibrating Screen Separator
Centrifuge: By spinning the manure, the centrifugal forces generated help
to speed up the gravitational separa-tion of solid particles. Centrifuge systems
can be very ef-fective at separating solids and can leave the solids with
relatively low moisture levels. Initial cost is high, however and the energy
requirement is quite high compared to most other systems.
Brushed screen with press-rolls: Figure 4 shows a system which is being used
successfully on several Ontario farms. In the first stage, manure is separated
using a screen. The screen is kept clean by a rotating brush which moves the
solids on to the next stage. Here, a roller presses more li-quid out of the
solids. The concentrated solids are then brushed out of the separator and
are transferred to storage.
Figure
4. Brushed screen with press-rolls
Submerged screen: Figure 5 shows an example of a "batch" system
of separation. The tank is filled with li-quid manure. Solid particles are
trapped on the screen as the liquid passes through. These solid particles
then act as an additional filter to capture even smaller particles. Even-tually,
the tank is drained, the solids are removed, and the process starts over.
The amount of solids removed, as with most systems, is highly dependent on
the rate that manure runs through the system.
Figure
5. Batch separator with scraped submerged screen.
Other designs: Several other separator
designs are on the market. Some of these include the vibrating screen with
screw press, cyclone, roller press, brushed screen, rotary screw press, and
the porous belt press. Each of these has its own merits which should be individually
assessed when choosing a system.
Nutrient
Analysis
To make the best use of nutrients in the manure,
a farmer must know their concentration. Tables of values are found in several
reference books. However, these have been found to vary considerably from
one farm to the next. Also, these values have been determined for liquid manure
and for solid manure, but not the separated li-quids and solids. The best
way to find out the nutrient con-tent is to get the manure analyzed. This
can be done for a nominal fee through the Agri-Food Laboratory.
Buying
a Separator
Those selling separation equipment encourage
farmers to picture the overall system of manure handling. It's easiest to
justify installing a separator when a system can be set up which makes use
of the strong points of the unit. For example, a typical system consists of
a separator near the barn with the liquid effluent pumped into an earthen
manure storage located in the middle of the farm. The storage is central and
irrigation of the liquid onto the near-by fields is quite convenient.
A separator may be installed as part of an existing system to help solve a
particular problem. If plugged pipes are a problem, for example, a separator
may help to eliminate the problem.
Because of the many advantages and disadvantages of separator systems, and
the variety of mechanical separators on the market, whether or not to install
a separator must be a very individual decision.
This
Factsheet was authored by: Ron Fleming, Agricultural Engineering Service. |
|