The difference between good and bad effluent

Beneath the surface
with David Law
Forward Farming

As the environmental consultant of Sustainable Water 2019 Ltd, Ashburton’s Mike Harrison is on the ground every day witnessing the challenges dairy farmers encounter with their effluent ponds.

And with so much riding on the ponds performing as they are intended, any issues need to be fixed as quickly as possible.

“One predominant problem we are seeing is a build-up of solids in farmers’ effluent ponds,” says Mike.

“As Canterbury has a high number of dual-pond effluent systems, where the solids are separated from the liquid before it is irrigated to pasture with a pivot irrigator, this issue can be particularly impactful.”

Mike says the only option available to farmers is to engage a pumping contractor to empty the pond and spread it on the farm.

“One client was required to pay $45,000 to have their effluent pond emptied, so you can see that the cost can be considerable,” says Mike.

“It’s also an ongoing cost; you can expect to have to de-sludge and empty your effluent pond once every five years.

“Around $2000-10,000 per year is a considerable sum to come up with, as it’s usually not budgeted for.”

Longer-term solutions

It’s clear that the way the majority of farmers are maintaining their effluent ponds is just not sustainable – financially or environmentally.

With that in mind, Mike contacted the Forward Farming team to investigate longer-term solutions to farmers’ effluent woes.

Although farmers work with effluent every day, few fully understand just how much it impacts their farm business.

Years of research and field work using Slurry Bugs has left Forward Farming with a clear idea of the composition of good and bad effluent, and what makes it so important in the first place.

We’ve found that a healthy effluent pond is clear and free from crust; and when irrigated to pasture, is light-colored-to-clear with very little odour.

In contrast, an effluent pond with crusting demonstrates the effluent within is tending towards poor. Underneath the crust is a section of “dead water”, and, at the bottom of the pond, a sludgy build-up.

Poor effluent, which contains elevated levels of methane and nitric oxide as unprocessed solids, is dark and strong smelling when irrigated to pasture.

What makes the effluent “good” or “bad” is determined by the pH of the pond, and what type of bacteria thrives within those conditions.

Good bugs

A pond with a pH of 7.4 is ideal to create a healthy pond in which aerobic, or “good” bacteria thrive, naturally digesting the ponds’ solids and creating a crust-free pond with clear, processed effluent that is ready for soil to readily absorb.

In contrast, a lower pH pond (under 7) encourages the dominance of anaerobic bacteria, resulting in crusting, thick, undigested effluent and blocked up irrigators.

It can be hard to “read” an effluent pond that uses a solids separator or weeping wall, so in these cases a pH test will determine if a pond is dominated by aerobic or anaerobic bacteria.

Also, it is important to note that stirring a pond does not change the biological make-up of the pond – it simply disrupts the job of the aerobic bacteria to eat the crust and later, the sludge.

If the good bugs aren’t present, you must introduce them in the short term – we use Slurry Bugs – and follow up with a review of the farm’s biology in order to improve the conditions in which the good bugs thrive.

This is a process the Forward Farming team specialises in and we are always keen to help.


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