Most wastewater systems in NC use gravity distribution. Parallel (equal) and serial distribution are the types of gravity distribution used. "Parallel (equal) Distribution" means the distribution of effluent to a set of nitrification trenches in which each trench receives effluent in equivalent or proportional volumes (plus or minus 5 percent). Examples of parallel distribution are distribution boxes and flow divider tees. "Serial Distribution" means the distribution of effluent to a set of nitrification trenches constructed at different elevations in which one (1) trench at a time receives effluent in consecutive order beginning with the uppermost trench, by means of a drop box, a step-down or other approved distribution unit.
Pump or siphon distribution can be accomplished in three ways. First, Pressure dosed," means an effluent pump or siphon is used to deliver effluent to the nitrification trenches using serial distribution. Second, "Pressure distribution" means an effluent pump or siphon is used to delivered effluent to more than one nitrification trench using parallel distribution or an equal linear loading rate per square foot of nitrification trench bottom area for each distribution opening or orifice, an example is a pressure manifold. Third, "Pressure dispersal" means an effluent pump or siphon is used to deliver effluent in a manner that assures no more than a ten per cent difference in the flow between the first and last orifices on each distribution lateral, examples are low pressure pipe and drip.
Parallel distribution is used in many wastewater systems installed in the Coastal Plain and the Piedmont and in a few systems in the Mountains. The most common type of parallel distribution uses a distribution box to divide the flow between the trenches. A distribution box or D-box is a box, usually concrete, with an inlet opening at the highest elevation and the outlet opening at a lower elevation. The D-box must be installed level on a solid foundation of soil or concrete. Flow adjusting devices should be used to insure the flow to each nitrification trench is as equal as possible. A variation on the standard distribution box is the tipping distribution box. This device uses a specially shaped pan to send small doses, usually about 1.5 gallons, to the nitrification trenches. When the pan is full it tips sending the dose of effluent to the trenches. The other type of distribution device is the flow splitter, a tee shaped pipefitting that allows effluent to flow from the higher pipe to the lower pipe across a weir. When the splitter is installed level the flow can be effectively split in half.
Serial distribution is used in most of the wastewater systems installed in the Mountains and in many Piedmont installations. Serial distribution is essentially one trench installed on different elevations. Step-downs and drop boxes are commonly used in serial distribution to separate the trench segments, and to insure that the higher trench completely fills up with effluent before flowing to the next lower trench. Step-downs are a dam of solid soil at least as high as the gravel or other trench media and at least 2 feet long. A solid pipe is connected to the perforated corrugated pipe in the gravel and is run over the dam and down to the next lower trench and connected to the perforated corrugated pipe there. The soil must be compacted around and over the solid pipe. Drop boxes is another way to connect the trench segments. A drop box is a box, usually concrete, with pipe openings at 3 different elevations. The highest pipe elevation is for the inlet pipe, the lowest elevation is for the pipe feeding the nitrification trench, and the middle elevation opening is for the pipe to feed the next trench down slope.
Pressure dosed systems are used in all areas of NC. A pressure dosed system can be used in situations where the nitrification field is a conventional (gravel) system or some type of reduction system that is comparable to a conventional system. A pressure-dosed system can be used with various types of chambers, large diameter pipe, polystyrene aggregate, multi-pipe, or tire chips. A pressure- dosed system should not be used with a Prefabricated Permeable Block Panel System.
Pressure distribution systems can be used in all areas of NC, and is commonly used with a pressure manifold or in some cases a distribution box. A pressure distribution system is used when the nitrification trenches are equal length or the tap sizes on the pressure manifold can be adjusted to deliver the same amount of effluent per linear feet of nitrification trench.
Pressure dispersal systems are used in all areas of NC, usually on sites with soil limitations. Pressure dispersal is a new term used to describe low-pressure pipe systems and drip technology.
Alternating nitrification fields can be considered a type of distribution. In this system two fields are installed at the beginning. Each field is seventy-five percent of what would be required in the typical installation. No other reduction in linear footage of nitrification trenches can be taken for using media other than gravel. An effluent diversion valve is installed between the tank(s) and the nitrification fields to switch from one field to the other. This design can be used with gravity or pressure distribution networks.
There are advantages and disadvantages to each type of distribution. Some of these advantages and disadvantages are listed in the table.
| Type of distribution | Advantages | Disadvantages |
|---|---|---|
| Gravity | ||
| parallel | ||
| distribution box | low cost, leveling devices are easy to install, low maintenance | unequal distribution
if leveling devices are not used, uneven flow if box tilts, usually trenches must be equal length |
| tipping d-box | more uniform distribution mini doses, slightly higher maintenance | small additional
cost, uneven distribution if box tilts, usually trenches must be equal length |
| flow divider tee | more uniform distribution, low maintenance | small additional cost, limitation on number (multiples of 2) and length of trenches must be equal |
| serial | ||
| step-downs | better distribution on sloping sites, can use unequal length trenches, low maintenance | more
care during installation, pipe over dam is very close to soil surface in shallow installations, upslope trenches are continually ponded |
| drop boxes | better distribution on sloping sites, can use unequal length trenches, low maintenance | more
care during installation, box will be close to soil surface or above soil surface in shallow installations, upslope trenches are continually ponded |
| Pressure | ||
| pressure dosed-serial, step-downs | better
distribution on sloping sites, some resting of trenches, can use unequal length trenches | intermediate cost, more care during installation, pipe over dam is very close to soil surface in shallow installations, upslope trenches are continually ponded, some maintenance, immediate repair if pump fails |
| pressure dosed-drop box | better distribution
on sloping sites, some resting of trenches, can use unequal length trenches | intermediate cost, more care during installation, box is very close to soil surface in shallow installations, upslope trenches are continually ponded, some maintenance, immediate repair if pump fails |
| pressure distribution-d- box | some resting of trenches | intermediate cost, uneven flow if box tilts, trenches usually must be same length, some maintenance, immediate repair if pump fails |
| pressure distribution- pressure manifold | uniform
distribution, some resting of trenches | intermediate cost, box
to protect manifold, difficult to use if trenches are unequal length, medium level of maintenance, immediate repair if pump fails |
| pressure dispersal-LPP | overcome some
site limitations, uniform distribution of effluent, unequal trench lengths, | higher cost, more difficult to design and sometimes install, higher level of maintenance and maintenance costs, orifices can clog, immediate repair if pump fails |
| pressure dispersal-drip | topography is not a factor, no media is required, overcome some site limitations, installation has little impact on site | highest
cost, design more difficult, small orifices have potential for clogging, higher level of maintenance and monitoring required, care in installation, immediate repair if pump fails |
| Alternating nitrification fields | Fields may rest,
extends operational life of fields | initial cost is higher,
valve must be switched or field in use maybe overloaded |
Please address any questions to Dr. David Lindbo.
This page created by
Roland O.
Coburn,
Research Technician I
on 2/17/03.