Guideline Index

Chapter 13: Using Dairy Effluent

13.4 Sampling and testing of effluent ponds

Sampling effluent ponds prior to the application of effluent is often recommended to provide a more accurate guide to the nutrient content than can be gleaned from published tables of ‘typical’ values. When sending samples for analysis the following analyses should be requested: Phosphorus (P), Potassium (K), total Nitrogen (N), ammonia-N and salinity (Electrical ConductivityEC). The ammonia-N content provides a good indication of N availability for plant uptake and use. Knowing P and K content will assist in formulating any subsequent fertiliser requirements for the paddock.

Any laboratory that routinely conducts water testing should be able to test effluent samples.

Any activities conducted in and around effluent ponds must be undertaken with extreme caution as effluent ponds can be very deep, with steep internal walls. It may even be difficult to locate the edge of the pond due to plant growth.

13.4.1 Procedure for sampling:

13.4.1.1 Sample source

As with soil testing, the key to effluent sampling is to take a sample that will represent the effluent being applied to the pasture or crop. In many cases, it would be more appropriate to collect samples from the final pond, the gate valves or even from the pump.

Direct application systems are best sampled at the sump or pump.

Farmers who use flood irrigation would sample at the gate valve, as this is the easiest place to test. It is best to allow the pump or gate valve to flow for at least 10 minutes before collecting a sample.

When sampling from ponds, the following applies. If you intend to desludge the pond, take samples from the sludge layer of the pond. As indicated in Table 13.7, the levels of nutrients at the bottom of the pond will be higher than the liquid fraction at the top of the first pond. If you are only removing the liquid fraction of the first pond without any agitation, take samples from that part of the pond. It would be best to take a sample when the first pond is being stirred. It is, however, unlikely that the pond will be stirred by a contractor, sampled and then stirred again and applied after the results of the test have arrived. If you are pumping from a second or storage pond, the nutrient gradient is relatively constant, so sampling depth is not critical.

13.4.1.2 Equipment required for sampling:

You will need the following equipment for sampling:

  • An extendable pole with a clamp at one end to hold a sample bottle.
  • A plastic bucket.
  • Three or four 1-litre, water-washed sample bottles.
  • Insulated carrier boxes.
  • Crushed ice.
13.4.1.3 Procedure

Collect 250ml to 500ml samples from various sites around the final pond or from the pump or pipe outlet, using the sample bottle clamped to the pole until 3 to 4 litres have been collected. Collection from ponds should take place close to the surface but should exclude any crust material.

As each sample is collected, place it in the plastic bucket. Hands and foreign objects should not contact sampled effluent or the inside of the bucket.

Each 1-litre, water-washed sample bottle should be filled to the brim with thoroughly mixed effluent from the bucket, unless the sample will be frozen prior to sending it to the laboratory, in which case, leave enough space below the top of the bottle to allow for expansion.

If samples are refrigerated, ensure they reach the laboratory within 2 days of sampling. If samples are frozen, 7 days is acceptable. To prevent unnecessary waiting and sample spoiling, the laboratory should be notified that the samples are being sent.