To the casual observer a drip disposal system looks like any other subsurface onsite system with a pump. The only hint that it isn't a conventional pump system is the control panel and Arkal disk filter (Item 1). Item 2 is a septic tank followed by a recirculation tank. Effluent passed through the septic tank into the recirculation tank and into the recirculating sandfilter (Item 3). After passing through the filter, about 1/3 of the effluent is pumped to the drip field (Item 4) and two-thirds is returned back the recirculation tank where it will go back through the filter again. On average, effluent goes through this filter three times. This produces a higher quality effluent than a single pass filter would provide. Not all drip systems require a sandfilter. The soil at this site wasn't capable of adequately treating the effluent so the sandfilter was used to provide additional treatment.
The photo on the left is a close up of the control panel and disk filter system. The disk filter is the purple and black object in the lower left side of the panel. There's a second filter located out of sight behind the first filter. These filters remove solids smaller than approximately 120 microns. This keeps the drip emitters, which are about 900 microns, from clogging. The filters automatically backwash on a regular basis and the solids from the filters are returned to the septic tank.
On the right is a section of drip tubing. The arrow is pointing to an emitter with a drop of water almost ready to fall off. The industry has known for 20 years that even distribution over the whole absorption field is important to long system life. Twenty years ago pumps were used to dose conventional drainfields with effluent. It was more efficient than a gravity system. In the mid to late 1970s, low pressure distribution became popular. It was a major stride forward in effluent distribution. Entire absorption fields could be evenly dosed; at least in theory. Over the years many improvements have been made to the design and operation of low pressure systems. In the early 90s drip disposal became available in the wastewater field. Now, not only could the absorption field be evenly dosed, but very small, frequent doses could be applied. Small frequent doses have significant advantages in both treatment and disposal. The drip emitter above literally oozes water at about 0.7 gallons per hour. At this rate, effluent can be applied at a rate that soils can absorb without ponding.
There are several manufacturers producing drip dispersal systems. There is a variety of opinion about how the systems should be designed and installed. Some products provide secondary treatment prior to dispersal, other simply filter the effluent. Some products boast about pressure compensating emitters while other products regulate pressure with valves. Which one is best? VDH believes that the important concept is to apply each site is different and the system designer need to match the system design to the site. These systems are not suitable for cookbook type designs. Properly, sited, designed, installed, and operated all of the systems are capable of providing safe and sanitary wastewater disposal. The real question is which one best suits your needs. GMP#107 covers the siting and design criteria acceptable to the Department.