Effluent Disposal Systems

Most households in rural areas rely on a septic tank system to treat and dispose of household sewage and wastewater.

Properly installed and maintained, this is a hygienic and economical way of disposing of household wastewater.  If neglected, it can pollute the environment, put you and your neighbour’s health at risk and be expensive to fix.  For communities relying on septic tanks, effective operation and maintenance will prevent the need for expensive sewage schemes.

Septic tanks and effluent systems have evolved over recent times from the small primary 1600 litre tank with single 30 metre soakage system, to 6000 litre and upward septic tanks with secondary to tertiary treatment; and wastewater disposal systems from large bore in ground soakage to small bore dosing with sub surface pipelines with the additional benefits of in ground irrigation.

Also available are composting toilet systems and grey water disposal with holding/settling tanks and subsurface irrigation/disposal systems.

The septic system

The septic system has two main parts: the septic tank, in which larger particles are removed from the waste and some decomposition takes place, and the disposal field, in which the effluent soaks into the ground through perforated pipes.

Wastes flow by gravity down the house drain to the tank.  The partially treated effluent flows out of the tank into the drainage field, where natural processes in the soil absorb and purify it.

This is known as a primary treatment system, where the sewage within the tank is settled, treated with bacteria and the effluent filtered as it flows into the effluent drainage system, with the contact in the ground providing the final treatment.

Secondary treatment or advanced wastewater treatment is basically the treatment of sewage twice by means of bacteria and high level filtration, with the effluent able to be pumped, siphoned, or gravity fed, but must be piped in contact with the earth either underground or under an approved mulch (e.g.: bark, compost etc).

Tertiary treatment or advanced sewage treatment is the highest level of sewage treatment and is able to cleanse the effluent sufficiently to enable uses such as toilet flushing, vehicle washing and fire fighting.  Final polishing with a UV system may allow a return of effluent to a water way.

What does the septic tank do?

The main job of the septic tank is to pre-treat the household wastes before they enter the drainage field.  Three processes take place in the tank:

• The heavier, solid particles in the sewage settle to the bottom of the tank, forming sludge.  Lighter materials, including fat and grease, float to the surface forming a scum layer.
• Bacteria living in the septic tank break down some of the organic solids into liquid components, helping to reduce the build-up of sludge in the tank.
• Sludge and scum are stored in the tank, rather than being allowed to flow into the drainage field where they would quickly clog the soil.

This is the traditional design of a septic tank.  Load conditions may require a more sophisticated tank design.

Where it all goes

When the effluent leaves the septic tank, it is only partially treated.  Final treatment is carried out by natural processes occurring in the disposal field.  The type of disposal field used depends on the soil conditions, slope of section, vegetation, climate and underlying ground structure.  Types commonly used within the Hastings district are:

Soakage Trenches
These are the most common form of disposal field, used where there is a layer of topsoil over clay.  Suitable for both flat and sloping sites.

Perforated pipe is laid in shallow trenches filled with crushed gravel.  Effluent flows out of the holes in the pipe and soaks into the surrounding soil.  The soil and bacteria in the soil clean the effluent.

Many newer systems have a distribution box which allows effluent to be diverted from one trench to another.  One trench can be allowed to recover while the other is in use.  They should be changed every three to six months.

Low Pressure Effluent Dosing (LPED)
This system being a specifically designed system is similar to soakage trenches in that the trenches are shallow (but can be deeper), with the effluent lines being large bore pipe work (25 to 32 mm) with holes in the pipe work of specific size and at design intervals. 

The effluent is either pumped or siphoned through these effluent lines achieving full discharge throughout each trench.  Effluent is absorbed into the ground and in some cases, evapo-transpiration to atmosphere.

Sub Surface Drip Lines
Similar in type to the LPED system, the sub surface drip lines are small bore (around 15 mm) with design holes at intervals and are laid under the surface of the earth or on top of scarified ground and covered with an approved mulch. 

The drip lines are generally pump fed from the septic tank, the effluent being of a high quality.  This system is suited to irrigation, but not permitted within human activity.

Other Systems
There are various other types of systems which can be used in difficult or special situations.  Your waste water designer or Building Consent Authority can advise whether another type of system would be appropriate.

Which septic tank system do I choose?

Regardless of the various types of sewer treatment systems, the following requirements should be considered when determining the type of treatment and disposal system to suit the property:

• Site location.
• Topography of the site.
• Site size.
• Soil conditions and type.
• Winter water table level.
• Hawke’s Bay Regional Council requirements.

Hastings District Council recommends arranging a qualified waste water designer or drain layer conversant with waste water systems, to discuss and design an appropriate system.  The Yellow Pages (under Drainage Contractors and Consultants or the Master Plumbers section) may provide assistance.

Alternative systems

Composting toilets and grey water disposal are an alternative to the septic tank.  Many types of these alternative solutions are available; each proposed system is assessed on the design and technical information supplied. 

Any supporting waste water design and calculations are necessary in assessing the grey water system.

There are a number of web sites and literature on these subjects that can assist in arriving at the type(s) of system that is best suited.  For example:

• Building Biology and Ecology Institute (BBE) www.ecoprojects.co.nz
• Sustaining Hawke’s Bay Trust www.shbt.org.nz
• Separett www.separett.com
• Sustainability www.sustainability.govt.nz
• Greenbuild www.greenbuild.co.nz
• BRANZ www.branz.co.nz

A Standard for composting toilets – Waterless Composting Toilets AS/NZS 1546.2.2008 – has been developed to assist in bringing an understanding of the requirements of manufacture, installation and disposal to all parties.
For a copy of the Standard, please contact Standards New Zealand.

• Standards New Zealand website

The New Zealand Building Code recognises these toilets as non water borne and defines them as a privy.  It further identifies that privies must be located three metres from a dwelling but can be attached by a veranda, covered walkway etc.

Caring for your sewer treatment system

• Owners are legally responsible for maintaining their onsite waste water systems in good working order.
• Have your septic tank pumped out on average every three years; those with Advanced Waste Water systems should contact your installer/maintenance person for advice.
• In septic tanks, keep the outlet filters cleaned periodically, do not discard them as this can lead to failure of the effluent field.
• Advanced systems may need a maintenance contract, to ensure fail safe operation from qualified persons.  Confirm with your installer.
• Use householder cleaners, detergents, etc., in moderation.
• Do not pour chemicals, paints or thinners (or the like) down any drain.
• Reduce the amount of water used in the household.  Fix leaking taps, toilet cisterns etc.  Also washing machines and dishwashers can use an extraordinary amount of water.  Where possible, fit low flow shower heads, dual flush cisterns.
• Do not empty spa pools, swimming pools or allow rainwater to flow into the sewage system.
• Do not use the septic tank as a rubbish dump (i.e. sanitary napkins, disposable nappies etc).
• A kitchen waste disposal unit is not recommended.
• Use cooking oils sparingly.  Cooking and vegetable oils are proving to become a problem with Secondary treatment systems where these oils are used in reasonable quantities.

Protecting the effluent field

• Ensure the effluent field is not subject to ground water flow paths from roofs, driveways, landscaping and hillsides above the effluent area.
• Maintain and protect all plantings that are part of the effluent soakage area; if able, mow the field area leaving the grass a depth of 40 to 50 mm.
• Keep all animals clear of the effluent lines, however in some instances, periodic light grazing with sheep is warranted.
• Where the effluent is so designed, swap each ‘soakage leg’ every three to six months.  Enquire of your drain layer or system installer the best method for undertaking this.
• Where possible, do not drive over the effluent system or allow a driveway to be built over them.
• Do not grow deep rooting plants or trees near an effluent system.  Your waste water designer can provide a list of suitable plantings for effluent areas.
• Do not let effluent collect or pond on the ground surface.

Hawke’s Bay Regional Council Resource Consent for effluent discharge to land or Permitted Activity Letter
All enquiries concerning consents should be referred to Hawke’s Bay Regional Council.

• www.hbrc.govt.nz

Please Note:
If Resource Consent is required from HBRC, it must be provided with your Hastings District Council building consent application.