Complete Filtration

There are three basic ways to do 100% filtration: negative pressure with a large number of filters, positive pressure using the squirrel cage-type fans to push air through a smaller number of filters but at a higher pressure or air conditioning. A number of facilities with 100% filtration using the 95% DOP filters initially were smaller units with excess ventilation or boar studs that are air-conditioned.

But now the notion of 100% filtration is being applied to larger units by use of a filter bank placed in front of a cool cell. Typically, to handle the large volume of air needed to pass through a cool cell during summer ventilation, a filter bank needs to be about three times the size of the cool cell pad, says Reicks.

“Air conditioning offers a distinct advantage in that it allows you to greatly reduce the amount of airflow into the facility, and thus, the number of filters that are required,” says Reicks.

For example, without air conditioning, barns generally run at 300-400 cfm (cu. ft./min.) per animal, maximum, in the summer through an evaporative cool cell pad. With air conditioning, this number can be reduced to as low as 6-20 cfm/animal, year 'round.

Bail Out Option

Because 100% filtration can be a costly option, Reicks reports it may be possible to bail out of the filtration system at a certain temperature for farms that have a relatively low risk of aerosol transmission of the PRRS virus.

“For these farms, there is really low risk when the outside temperature is above 80-85°F. First, PRRS doesn't survive as long at high temperatures. Second, if the temperature is that warm outside, normally it means the sun is shining, so you have some added benefit of ultraviolet light working against the virus,” explains Reicks.

With the bail out system, air is 100% filtered up until the last couple of stages of ventilation turn on. Then a door opens near the filters, allowing air to bypass the filtration system. When the air temperature cools down, the trap door closes. The door can be operated by an actuator or curtain controller and tied to the thermostat.

“The advantage is that it reduces the number of filters you need at a time when, if you are several miles away from other pigs and there are not many pigs in the area, the risk is really low anyway,” he reports.

Another option for the bail out system is in farms where all of the air comes through ceiling inlets. Reicks says an extra row of inlets can be installed at a fairly low cost. When actuated, these inlets can also be tied into the ventilation system to provide a bail out option during periods of higher temperatures. These unfiltered inlets would open up when the outside temperature reaches the 80s, for example, and then close as the temperature drops again.

Positive vs. Negative Pressure

Overall, for producers with existing, conventional hog barns considering air filtration, Reicks says the 95% DOP filters can still be used with some building modifications. For these negative- pressure systems, producers need a filter bank that is about three times the size of their existing ventilation system in order to “suck the air through the system,” he explains.

A positive-pressure system requires fewer filters, but larger fans are needed to force the air through the system. “If we can push air through filters at higher static pressure, then airflow can be maintained at its current level. However, in order to push the air through the filters at high static pressure, a different type of fan is needed,” says Reicks.

He suggests a squirrel cage-type fan as being much more efficient at moving large amounts of air against a high static pressure. These are the types of fans used with grain drying equipment.

Reicks says these fans can be placed in front of a filter bank to drive air through the filter bank at high static pressures and prepare it for entry through an evaporative cool cell pad into the building.

Although negative-pressure systems cost less than positive-pressure systems, they do create some added challenges:

• Sealing up everything is a big concern, including around doors and windows. Duct tape and plastic can be used to seal doors.

• Doors must not lead directly into the rooms. “We have a rule on filtered sites that no single doors can be opened. Culls and deads have to be taken through isolation or through a hallway where doors of the barn are closed,” he says. This means that some type of double-door system must be installed so no animals are exposed directly to outside air.

• Back-drafting of fans should be avoided. Make sure fans don't stick open and air doesn't blow back into the rooms. Delay opening of wall fans in winter. Check wall fans at least once a week to ensure that they are in good operating order and are not stuck in the open position.

• Close off fans not in operation. When some fans are not in use (i.e. winter), use garbage bags or other types of covers to prevent back-drafting.

The take-home message for producers is if they live in a hog-dense area, they need to have their hog farms 100% filtered if they want to stay PRRS negative, Reicks comments.

Regardless of the size of the farm, adding filter banks in front of the cool cells is the easiest way to go.