What is effluent?

On dairy farms, the liquid waste stream from milking sheds and associated yards is generally referred to as effluent. It consists of the manure and urine excreted in the milking shed and yards, mixed with the wash-down water used to remove the waste from these areas and concrete yard runoff following rainfall. Consequently, it contains dissolved and suspended nutrients, salts and organic matter. Detergents and small amounts of spilt milk may also be present in effluent.

It should be noted that large quantities of milk should not be disposed of through the effluent system. Various options for milk disposal on farm are outlined by Crocos (2000).

Other intensively stocked farm areas such as feed pads, earth yards, loafing areas and regularly used laneways are generally cleaned by dry scraping rather than wash down. However, runoff from these areas following rainfall also contains relatively high concentrations of nutrients, salts and organic matter. This runoff is also commonly referred to as effluent.

Why manage effluent?

There are sound practical and economic reasons for paying close attention to effluent management:

• effluent contains valuable water and nutrient resources that can be utilised to promote crop or pasture growth, and potentially reduce inorganic fertiliser and irrigation water requirements
• effluent may be harmful to the quality of underground and surface water resources, and to the chemical and physical characteristics of soil, if it is not managed appropriately
• every Queenslander is subject to the general environmental duty defined under the Environmental Protection Act 1994. To demonstrate compliance with this duty, all dairy farmers need to take all reasonable and practicable measures to manage their effluent responsibly, thereby preventing adverse environmental impacts.

Types of effluent management systems

The two major types of effluent management systems are continuous application systems and treatment and storage systems.

Continuous application systems

These systems are not designed to treat effluent and have limited storage capacity. Consequently, they rely on regular collection and application of effluent - usually twice daily following each milking. The effluent is generally collected in a concrete sump and applied directly to pasture.

The main types of continuous effluent application systems are:

• sump and gravity flow (generally through a moveable hose)
• sump, pump and moveable sprinkler
• sump and effluent tanker.

To protect pumps and prevent pipe blockages, each of the above systems needs a stone trap, screen, or trafficable solids trap to remove coarse solids and foreign material from the effluent stream before it enters the sump.

Advantages

The advantages of this system include:

• higher effluent nutrient concentrations compared to effluent treated in a pond, where nitrogen is volatilised (released as ammonia gas) and sludge settles out
• application of solid wastes to land with the liquid effluent; so no pond sludge to deal with separately
• low capital cost for sump and gravity flow system, though a medium-to-high capital cost for pump and sprinkler, and effluent tanker systems.

Disadvantages

The disadvantages of this system include:

• the regular shifting of sprinkler/hose required every few days to avoid overloading land areas with nutrients and salts
• limited storage capacity (often less than one day's washdown, flushing and hosing volume), which means that effluent is inevitably applied to wet land during periods of prolonged wet weather. This can potentially result in contaminated runoff entering watercourses and/or leaching into groundwater
• the limited area over which effluent can be applied
• the need for the effluent application area to be well removed from watercourses
• the reliable pump, capable of handling high levels of solids, required for sump-pump systems. Pump breakdowns must be fixed very promptly due to the very limited storage capacity
• the lack of scope for recycling of effluent for yard flushing purposes
• the fact that tankers may be difficult or impossible to operate in wet conditions on most soil types.

Treatment and storage systems

From an environmental perspective, effluent ponds are generally preferable to continuous application systems in drier areas of Queensland (average annual rainfall less than approximately 1200 mm).

These systems employ one or more ponds (generally one or two) to treat the daily inflow of effluent from the milking shed and yards, and to store both the liquid effluent and solids (sludge) that settle out of the effluent. Pond systems can also collect, treat and store runoff from concrete and earth yards, and, in some cases, feed pads and regularly used laneways. The liquid effluent is stored until it is either irrigated onto crop or pasture or recycled for yard flushing purposes.

A number of ponds may be constructed in series to treat and store dairy effluent. The first pond in such a series is generally referred to as the primary pond and the second pond as the secondary pond. While systems employing three or more ponds are uncommon, the third pond may be referred to as a tertiary pond. The quality of the treated effluent in the final pond generally improves as the number of ponds in the effluent management system increases.

Sludge accumulates in the primary pond and is removed at regular intervals. Primary ponds are commonly designed to store 1-10 years' accumulated sludge. The sludge storage capacity generally depends on the intended method of sludge removal. For example, if a farmer wishes to employ a contractor with an excavator to remove the sludge, they may prefer to limit desludging operations to a frequency of once every 10 years. Alternatively, if the farmer has ready access to a vacuum tanker, they may be prepared to pump out and apply the sludge much more frequently, perhaps annually.

Regardless of the number of ponds in the effluent management system, the following three storage/treatment volume components must be provided:

• active treatment volume to maintain the necessary bacterial population to treat and break down the organic matter in the effluent stream
• sludge storage volume to store the solids that settle out of the effluent during treatment
• wet weather storage volume to store liquid effluent during periods when the land is too wet for effluent irrigation, or until the timing of effluent irrigation suits other farm management considerations.

In a single-pond system, each of the three treatment/storage volumes described above must be provided in the primary pond (see Figure 1). In a double-pond system, the active treatment volume and sludge storage volume must be provided in the primary pond and the wet weather storage volume in the secondary pond (see Figure 2).