Land Disposal: Giving My Wastewater Somewhere To Go

Back in December, we shared an article called Discharge Options: Where does your wastewater go? The purpose was to help you understand the options for effluent disposal. Then, in January we discussed Soil Monitoring: Why Should I Care About My Wastewater? (Beyond the Obvious!) The purpose of that article was to discuss the effects that wastewater application can have on the receiving environment. Now, in the next article in the series, we’d like to address land disposal and the points to consider when deciding on which method is best.

First of all, let’s look at where the wastewater actually ends up when it is discharged to land:

The  diagram above shows the potential movement of water after application to the land. When disposing of wastewater, it’s crucial that the irrigation rate does not exceed the soil and plants’ capacity to absorb water. If you do exceed that capacity, it leads to runoff over the surface of the land, which is a breach of most irrigation licences. However, enough water should be applied to keep plants and crops from wilting. The finer details of trying to strike this balance can be found here.

The first questions to ask when considering land disposal are:

• How much land can be dedicated to wastewater disposal?
• What is the wastewater quality like?
  • For a new system, you need to assess the irrigation area against the proposed effluent targets. If an existing system is being assessed, then you may be able to supplant this data with actual water quality data, but care needs to be exercised that the system needs to be able to accommodate the maximum effluent quality. Applications are generally assessed by the regulator, so the design basis needs to be sound.
• What is the soil and plant quality like? (This will require soil testing.)
• Is the land reserve located near public access areas (e.g. footpaths, roads)?
  • This will help inform the best land disposal method.
• Are there any setback requirements from lot boundaries, existing water courses or other sensitive environmental receptors?
  • This might reduce the actual land area available.

The land required depends on the amount of wastewater generated, the soil and plant characteristics, and the local climate. Environmental permits may set disposal rates, maximum flows, or both, which will limit the amount of water that may be disposed.  The recommended rates are dependent upon the soil characteristics, and there is a significant amount of data published on acceptable irrigation rates. The final rates are generally dependent upon a combination of yearly rainfall, and the soil and plant capacity.

The establishment of design irrigation rates, the size of the disposal area and the peak irrigation rates are generally established as part of the original application; as such if a new application is made, care needs to be taken to ensure that sufficient flow can be directed to the effluent irrigation area to accommodate irrigation disposal.

There are a range of systems that can be used to get the wastewater into the soil. Common land disposal methods referenced in Australian Standards are:

• Spray irrigation;
• Drip irrigation;
• LPED (low pressure effluent distribution) irrigation;
• Trenches;
• Beds;
• Mounds.

All of the above are relatively simple to install and require little manual operation and maintenance effort in domestic applications. Spray irrigation is very common, and consists of a pumping station that sends wastewater through a piping network to sprinkler heads, evenly distributing the waste onto the irrigation area. However, since the wastewater becomes airborne, this is not the best solution if there is public access near the irrigation area. Depending upon the quality of the wastewater, a subsurface option may be required.

In these situations, drip or LPED irrigation might be a suitable alternative. These systems also require a piping network but slowly release wastewater just underneath the soil (or above for drip irrigation) through pipe perforations or drip attachments. This minimises the risk of exposure to bacteria that may be present in the waste.

Trenches, beds and mounds are sub-surface systems, which can be gravity-fed or pumped depending on the site. These systems use an aggregate or sand to help distribute wastewater evenly throughout the surrounding soil. Trenches, beds and mounds are rarely used for systems larger than a household or small commercial operations and can be quite common for septic tanks or small AWTS’s (aerated wastewater treatment systems).

Another requirement is typically storage for wet weather events. Irrigation should not be applied during rainfall events as this leads to the risk of wastewater running off the surface of the land, washing contaminants out of the irrigation area. To solve this problem, wet weather storage tanks are used to hold wastewater until the soil can take the wastewater again.  Figuring out what size tank is needed is generally conducted using MEDLI modelling (read about MEDLI here), with actual sizing depending upon the frequency and intensity of rainfall events.  Generally 3 days of storage is the minimum required by the regulator, although this can be too small in some instances.

Some other issues may arise from improper irrigation practices, namely soil and plant degradation. This is further explained in  here.

Finally the irrigation system has to be controlled. Small systems often utilise simple timer controls for the irrigation pumps, with a rain-gauge cut-out to prevent irrigation during rainfall.  Larger systems can be controlled via soil-water moisture sensors that prevent irrigation if the soil is saturated, with automatic zone rotation and sophisticated pressure compensation and leak detection equipment. Generally speaking, the larger and riskier the irrigation area, the more control will be required to ensure the system works properly (and to make sure the regulator is happy with your system).