Utilising effluent pond sludge

20 Nov, 2018 04:00 PM
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Sludge can be removed from ponds using an excavator.
There are some indicators for when a pond should be desludged.
Sludge can be removed from ponds using an excavator.

The build-up of solids or sludge in an effluent system can be problematic as it can reduce the storage volume in single-pond systems and increase transfer of solids into the second pond or storage pond(s) in multiple-pond systems.

On most farms, there are systems in place to manage the liquid effluent fraction, often through dedicated or existing irrigation infrastructure. However, solids are typically more difficult to manage and usually require the farmer to engage a contractor with specialist equipment.

Effluent ponds are designed to accumulate a percentage of their total volume as 'sludge storage' and require desludging (e.g. cleanout frequency) when the 'sludge storage' volume has been filled. An accredited effluent system designer will provide an indicative cleanout frequency for both single and multiple pond systems.

The following parameters determine the cleanout frequency:

The size of the pond.

The number of cows being milked.

The time the herd spends on areas draining to the effluent systems e.g. dairy shed, yards, entry-exit laneways and feedpads.

The effectiveness of the T-piece or liquid transfer between the first and second pond in a multiple-pond system.

The effectiveness of other solids management prior to the pond system e.g. a trafficable solids trap, run down screen, screw press.

If there are changes to the parameters above, the clean-out frequency will be affected. For example, doubling the milking herd from 200 cows to 400 cows will increase the time the herd spends on areas draining to the effluent system. This will result in a higher manure loading (e.g. more solids) into the pond system and decrease the time between cleanouts.

There are some indicators for when a pond should be desludged. In a single-pond system an accumulation of sludge reduces the pond's capacity to store effluent over the wetter months of the year when irrigation is not recommended.

If the final-pond in a multiple pond system is starting to develop a floating crust or has gas bubbling, then desludging may be required to improve effluent quality for irrigation or recycling for yard washing.

There are several ways to remove solids from an effluent system. These include pumping with agitation, vacuum tankers, and excavators. The volume of sludge in the pond(s) and availability of other manure (e.g. dry-scraped from a feedpad or trafficable solids trap) will influence how sludge is managed.

Local contractor slurry services will provide pumps, stirrers and tankers. The are several machinery dealerships and farmers that dry-hire vacuum tankers, stirrers, manure spreaders and high-horsepower tractors. Costing should be considered when selecting the best option for the farm, as larger ponds can be time-consuming and expensive to desludge.

Agitation and pumping

Agitation assists to raise and mix accumulated nutrients and salts in the lower layers of sludge, which will reduce pond performance over time. Agitation or stirring also assists in breaking down solids, creating a more liquid consistency making it easier to pump. Pumping in freshwater or recycling effluent from the final pond will improve agitation and assist the breakdown of a floating manure crust.

A specialist sludge pump should be used that can efficiently pump liquids with a high suspended solids content.

Vacuum tanker

Vacuum tankers or slurry tankers can be used to suck sludge from ponds under agitation and transport it for direct application through dibble bars or splash plates. Tankers are typically 8000-15,000 litres. When assessing the cost-effectiveness of different contractors, it is important to seek a total cost per megalitre applied (i.e. $/ML) and not their hourly rate. Experienced contractors will often operate larger tankers that have a more expensive hourly rate, but it is likely their rate will include agitation and a quick-fill setup at the pond that will result in the cheapest cost per megalitre applied.

Excavator

An excavator can be used to dig out the sludge. To make the sludge easier to handle with an excavator, as it often acts like a liquid, reducing the water content of the sludge can assist.

Use the normal liquid distribution system to remove as much liquid as possible; for gravity systems this can be difficult. Where possible the effluent stream may be diverted around the solids pond into another pond or irrigated directly, if conditions are suitable, for a short period to assist further drying.

The solids can be applied directly to maximise the use of nutrients or stockpiled on a drying or evaporation pad and spread at a time convenient to the farm. Stockpiling sludge will result in the loss of nitrogen through volatilisation.

Manure or sludge needs to be stockpiled away from waterways on an impermeable, bunded surface. Run off from this area should be diverted back in to the effluent system.

The nutrient content of the sludge, along with liquid effluent, can vary widely from farm to farm.

Taking a sample of the effluent for analysis is the best way to determine the nutrient content. It is best to take samples when the pond has been agitated, if agitation is used to remove the sludge, as the nutrient within the sludge changes with depth. This may be completed when the contractor is there applying the effluent. This will not influence the application this time but will provide a better indication when next utilised compared with the use of 'industry averages'.

Applying sludge to areas of the farm low in nutrients will assist making the best use of the nutrients contained in the sludge. Preferably, apply sludge to areas that do not receive liquid effluent to increase the distribution of nutrient across the farm.

Modify the application rate of other nutrient sources to account for the nutrients within the sludge to produce the best returns. Regular soil testing is important to monitor farm nutrient distribution.

Sludge or manure can be applied to established pasture and crops, or prior to cultivation. If applying to established pasture or crops, ensure the application rate does not smother the plants. Our research carried in South West Victoria indicates a sludge application rate of 5-10 millimetres on established pastures provided the best returns. Higher application rates can be used when applied prior to cultivation before sowing forage crops or pastures.

Not all the nutrients in the sludge will be available within the first year. Our research showed an increase in dry matter production three years after the application of sludge. Apparent nitrogen recovery or the proportion of the total nitrogen applied that is taken up by pasture was found to be 40-50 per cent in year one, 10-20pc in year two and 5-10pc in year three.

The Dairy Australia Nutrients from Effluent and Sludge Calculator is one tool that can assist farmers and agronomists determine an appropriate application rate for dairy farm sludge and/or effluent. It is available at website .

After application, a withhold period of three to six weeks should be observed to reduce palatability issues and minimise the impact of increased nitrate levels. Sludge can also interfere with the mineral balance of the forage grown for a few months post application. It is suggested to avoid grazing classes of stock susceptible to milk fever.

Effluent ponds can have steep sides and be overgrown with grass and weeds. Implement appropriate safety controls for staff and machinery before undertaking pond maintenance activities. D

For more information on using and management of dairy effluent contact a Agriculture Victoria dairy extension officer.

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