The Super Siphon LPED Design
The Super Siphon has been developed as part of the Low Pressure Effluent Distribution (LPED) disposal system as outlined in the New Zealand Auckland Regional Council Publication TP58. TP58 has been developed as a guideline for the design and installation of on-site land disposal of domestic wastewater in the Auckland region and has become the template for on-site waste design New Zealand wide. The design criteria used in TP58 are similar to criteria from regulatory authorities in Europe and North America. Numerous systems have been installed with the Super Siphon and its predecessors the AFS series of siphons. A review of TP58 is advisable prior to designing with the Super Siphon LPED system. The system components are listed below and are designed and sized using a proprietary spreadsheet.
Design spreadsheet [Excel software]
A well designed Low Pressure Effluent Distribution (LPED) system is one in which the layout of the field and sizing of the components are all specifically designed. Some of these design requirements are as follows:
- The dosing volume from the tank has to be greater than the volume of the disposal field to entirely fill the field for even distribution.
- The disposal field lateral lines need to have sufficient pressure to squirt and clear blockages or root intrusions but not so much pressure that the system is prone to leaks.
- The entire system has to be sized to allow for frequent doses with rest periods in between.
- The system needs to be balanced to ensure even distribution throughout the network no matter what size or how long the individual distribution lines are or what elevation they are laid at.
- The system needs to be designed to ensure that the bottom lateral line does not become overloaded by effluent within the network draining to the lowest point.
All of these requirements are defined by the physical layout of the tank and field including the relative levels from the tank to the lateral lines, the levels of the lateral lines to each other, the distances involved between components, the size of the individual components selected and the frictional losses each component contributes for a given flow rate. The Vortech design spreadsheet allows the user to input all these values and then vary components to achieve the best design possible. While the spreadsheet has been designed to be as foolproof as possible, it should only be used by someone familiar with on-site waste design and the hydraulic principles involved.
Two chamber, typically concrete septic tank
This tank can be obtained from a range of vendors but must meet the requirement of AS/NZS 1546.1:1998. The primary chamber receives and treats the solids while the liquids pass through a biological filter into the dosing chamber. The secondary chamber in the septic tank is a settling and dosing chamber with the Super Siphon mounted within it. In normal operation, as the dosing chamber fills the effluent rises above the siphon bell and when the siphon doses the effluent flows through the siphon and out to the disposal field.
A biological filter between the first and second chamber
The filter helps retain solids and the microbial mass in the primary tank to assist in primary treatment. Typically a filtration mesh size of 2mm is utilized in the primary chamber, this helps keep the lateral “squirt” lines from clogging as the filtration size is smaller than the typically 3mm diameter squirt holes used. Other squirt holes normally between 2.5-5mm diameter may be selected.
A Super Siphon gravity dosing siphon
The Super Siphon controls the dosing of the treated effluent to the disposal field. Once primed for the first time it should operate indefinitely with no adjustment or cleaning necessary. Not all siphons are created equal… other siphons on the market can stall (stop working and allow continuous trickling) if the flow rates are too low or too high. The Super Siphon has been designed to prevent this and can be completely stopped mid dose without stalling. The calculation spreadsheet has been specifically designed for the Super Siphon flow characteristics and should not be used with any other siphon.
Mainline to Disposal Field
The effluent travels down the mainline from the siphon to the manifold at the top of the disposal field. The size of the mainline is dependent on the flow rate, slope, and length. On sites with shallow slopes a large mainline will reduce frictional losses and preserve head for field distribution and “squirt”. On steep slopes a smaller diameter mainline can be used to “choke” the flow and bleed off excess head prior to the effluent entering the field. Where the mainline is to be reduced from the 50mm diameter outlet, at least 250mm of 50mm diameter pipe should be placed at the outlet of the siphon to allow for unimpeded flow from the siphon at startup.
The manifold is placed just above the top line of the disposal field and divides the effluent evenly between the laterals of the field. At the top of the manifold is a shutoff to stop the flow in the mainline for servicing and then a threaded “T” with plug is placed so that fresh water can be pressure fed if necessary into the manifold to flush the manifold and laterals. The manifold should be fabricated specifically for each job with the number of outlets to match the number of laterals. It is very important when plumbing the manifold that the lowest outlet of the manifold feed the highest line in the field. This prevents drain back at the end of the dose favoring the lower field lines and over wetting or flooding the bottom of the field. The bottom of the manifold is capped. Typical manifolds are shown below, one for centre feed fields and one for side feed fields.
The non-perforated delivery lines run from the manifold to each of the laterals. The further downhill the lateral, the longer the delivery line, the more head loss in the delivery line which helps compensate for the head gain of the lateral at the lower elevation. Hence the delivery lines do assist in balancing the field. Delivery lines are typically the same diameter as the laterals or squirt lines. They can be of smaller diameter to throttle back head if needed to a particular line. The downside is that delivery lines of differing diameters are more complicated and easy for the drainlayer to overlook during installation. A clay dam should be constructed around the delivery lines to insure that effluent from one lateral trench isn’t able to flow through the delivery line trenches to another trench, short circuiting the design.
Laterals or “Squirt” lines
The laterals consist of selected diameter Low Density PolyEthelene Pipe (LDPE) pipe, with squirt holes at calculated centres, laid within perforated drain coil and laid dead level to the contour of the land. The diameter of the lateral line may range from 15 to 50mm diameter, however, it will be found that smaller pipes have higher head losses and the larger pipes contain too much volume to fill after each dose unless the squirt holes are fitted with irrigation whisker risers so that the pipe remains full after dosing. The length of the lateral required should not permit a drop in squirt volume from the first to the last hole of more than 5% to ensure even distribution and preferably the last squirt height should not be less than 1.4 metres. Within these constraints the length of the lateral is a function of the head at the first hole, the diameter of the lateral, the number, size and spacing of the holes. A spreadsheet analysis is required to accurately determine the maximum length of the lateral line. However, as a general rule 25mm diameter lines up to 30 metres long will be about optimum. As the squirt line discharges the effluent will pool in the perforated coil and drain into the surrounding soil. These laterals can be laid in 200x200mm trenches, backfilled with drainage metal or bark and overtopped with soil or can be pinned to the ground surface and covered with leaf detritus, bark or mulch. The ends of the lines should have a plug or valve that can be opened to allow the laterals to be flushed. The flushing valves shall be placed in such a way (i.e. in a PVC “mushroom” or box) that they can be easily located. The squirt line has a specific number of holes drilled at a defined spacing based on the results of the calculation spreadsheet. The holes should be drilled carefully to remove all swarf from the hole that may impede the flow of effluent. It is important that the disposal field be fenced from stock to prevent damage and in the case of surface laid systems that children and pets are kept out as well. It is very important to check a LPED design for correct "squirt" during commisioning to confirm the design and installation. See the photo below of a typical LPED line squirting prior to being slid inside the perforated pipe and the trench backfilled.
Super Siphon LPED Design Methodology
When presented with a new job a typical design methodology is as follows:
Can an LPED system be used? Use the Super Siphon spreadsheet.
Decide on the Design Occupancy and Flow Allowances based on the TP58 tables shown on the first part of the spreadsheet and then input the section size in square metres. Is the lot size to discharge ratio (m2/litre/day) greater than 3? This is the ARC minimum requirement for primary systems.
Can a Super Siphon system be used?
Decide on tank and field locations. Is there a vertical fall of at least 0.5 metres between tank and field? If so it may be possible in the absence of aggressive root growth plants and with special care and design to use a Super Siphon. Generally however a difference in head of 3 metres between the top of the tank and the first lateral is desired to ensure that a self cleaning squirt height of 1.5 metres can be achieved using a 3mm diameter squirt hole size. If not, the site may be appropriate for a pump dosed LPED system.
Specific Design of the System
What is location, shape, use of field? Look at boundary/flow path offsets, orchard or landscaping etc.
Check soil and water table depth in area of the field and categorize.
Calculate length of field required
Design lateral lines… 30m lateral runs in one direction maximum if possible
If greater than 30m does field have to be split
Size mainline and time of dose in spreadsheet
Adjust squirt hole sizes if needed to keep maximum squirt hole spacing below 3 metres.
For more information about design of LPED system or obtaining a copy of the design spreadsheet contact Ashby Consulting Engineering.