Design

The CQUniversity On-Site Wastewater Soil-Bed Treatment and Transpiration Reuse System in Relation to the Australian/New Zealand Standard On-Site Domestic Wastewater Management AS/NZS 1547:2000 and the Queensland Department of Natural Resources Interim Code of Practice for On-Site Sewerage Facilities Q.G.G. 2/7/99 No. 65

Introduction

The AS/NZS 1547:2000 and the Q.G.G. 2/7/99 No. 65 are the most current legislative acts responsible for on-site wastewater management. The Queensland Interim Code closely parallels the AS/NZS 1547:2000 and is designed to follow its recommendations in the majority of cases. The most significant difference between the two documents is that the AS/NZS 1547:2000 deals only with systems up to 10 equivalent people (E.P.) whereas the Qld Interim Code covers systems up to 20 E.P.

The CQUniversity is currently testing a new on-site wastewater treatment and reuse system. The final aim of this project is for the trial system to gain type certification under the Qld. Interim Code and the AS/NZS 1547:2000.

The CQUniversity System

The CQUniversity on-site wastewater treatment and reuse system is designed to be an ecologically and economically sustainable. It uses an innovative mix of existing wastewater technology and new system design. It is intended for use on domestic blocks, small amenities, light commercial and industrial situations not connected to town sewage systems.

The wastewater is primarily treated either in an all-waste septic tank (combined black and grey water), or separately with the blackwater treated in a septic tank and the greywater in a large vertical greasetrap.

The primarily treated effluent is then collected into a holding tank. Inside the holding tank is a timed pump that delivers specific dosages of effluent to a self-contained land application treatment and reuse system.

A venturi valve aerates the effluent as it is pumped to the land application system. The land application system is based upon an enclosed concrete channel. The channel is made up of concrete troughs 3 metres long, 1 metre wide and 0.4 metres deep.

The concrete troughs are recessed into the ground approximately 200 mm. After mounding, this allows plants to put roots outside of the trough into the natural environment while also preventing rainfall intrusion from runoff.

A 50 mm slotted DWV PVC pipe that is covered in filter sock runs through the bottom of the trough. This pipe enters the trough at the 150 mm level, drops down to run along the bottom and then rises again to exit at the 150 mm level. All slots in the pipe face the bottom side of the trough, and all entry and exit points are sealed with o-rings.

The 50 mm slotted DWV PVC pipe is surrounded by and covered with an aggregate that meets the requirements of AS 2758.1. The final depth of this aggregate is 100 mm. The aggregate is expected to act as a gravel bed filter that incorporates an active biofilm layer.

On top of the aggregate layer is placed filter-cloth/terra-firma matting. This acts as a physical barrier between the aggregate layer and the soil bed. The soil bed is placed above the filter-cloth/terra-firma matting to a depth of 500 mm. This makes the soil-bed 200 mm higher than the top of the trough. The filter-cloth/terra-firma matting prevents soil infiltration and the clogging of the aggregate layer. The soil-bed itself is a treatment technique for the effluent.

Fill is used to heap up either side of the trough so that a mound is formed. The mound normally extends 1000 mm either side of the trough. The mounding reduces the risk of runoff and rainfall intrusion.

The soil-bed is completely covered by 100 micron black plastic. The plastic is used to reduce rainfall infiltration. Holes are cut in this plastic so that a variety of trees, shrubs, and climbers may be planted. These plants reuse the effluent water and its associated nutrients for transpiration and homeostasis. Phytoremediation is the major treatment technique.

The black plastic is covered with a thick layer of mulch. The plants can put roots down into the trough to gather water and nutrients, however they can also put roots outside of the trough into the surrounding aerated soil.

Any effluent water not absorbed into the soil or used for transpiration or homeostasis by the plants is returned to the holding tank from which it was pumped. From there it can be pumped through the enclosed trough channel repeatedly until it is used. This markedly increases the detention time of the effluent water. As it repeatedly travels though the system it is constantly being treated through a variety of techniques until it is used.

The plants under standard conditions use more effluent water during daylight hours than the soil can absorb. Studies on the system have shown that the soils recharge effluent water during the night when the plants are not as active.

The holding tank incorporates several design features in addition to the pump. An emergency soakage drain is attached to the holding tank. This means that if the pump fails the effluent water is disposed of underground, rather than the tank overflowing. The emergency soakage drain can also deal with unexpected hydraulic surges.

Note that in some cases an emergency soakage drain has not been installed, in these situations the emergency overflow is connected to an aerated wastewater treatment system (AWTS). The pump timer has an alarm and buzzer fitted to alert the property owner if the pump fails.

A low-water feed is attached to the holding tank. This is float operated and incorporates a breakwater/backflow prevention tank. If the water level in the holding tank drops below a predetermined level the low-water feed is activated and water is added to the holding tank. Water can be added from a reticulated supply, rainwater, or borewater, depending on the site.

The holding tank is designed to act as a second anaerobic treatment chamber. This increases the detention time of the water, thereby improving the quality of the effluent water. The holding tank is also designed to have wet weather storage capacity.

CQUniversity is confident that this trial system meets the requirements of the AS/NZS 1547:2000 and the Qld Interim code.