Effluent Tanks - what you need to know

Smelly business effluent, most people try to avoid it at all costs but we all know we need to store it – somehow.

So what do I need to know about effluent storage?

As we have covered elsewhere in this series, silage effluent is a nasty substance that has one of the highest Biological Oxygen Demands known to the world of pollution. As a result of its deadly reputation, there are (unsurprisingly) a raft of rules and regulations that control your control of this liquor.  These rules are detailed in the SAFFO regulations, but their origins and details date back to the Control of Pollution Regulations 1991 set out in the Water Resources Act

That’s enough legalise – what do the silage effluent regulations mean? These rules give you the tools to calculate the size of tank you need and how it must be built. So let’s start with what size of tank and do the maths.

The rules states you need 20.0 litres of effluent storage for every cubic metre of clamp space you have for the first 1500 cubic metres of silo. For every cubic metre of clamp space over 1500m³, the rules require 6.7 litres of effluent space. So for a 2000 tonne silage clamp you would have about 2750m³ of clamp space. That’s equates to 1500m³ basic and an extra 1250m³ in the eyes of the regulations. So we have 20 litres for the first 1500m³ giving 30,000 litres and 1250m³ requiring 8,375 litres on top.

So a total of 38,375 litres or 8,442 gallons of storage capacity is required for a 2000t clamp.

This seems a lot, do you really need that much storage?

Can I reduce the effluent storage requirement to suit cuts of silage? That’s two questions with two answers - Yes you do, and no you can’t. The SSAFO regulations are set out in law, they are not guidelines they are rules, if you don’t stick to these, you will be prosecuted when you are caught. The rules understand that a livestock farmer is not going to fill all the silage in one hit so the volume calculation allows for this.

The thinking is that you are likely to fill the clamps in (roughly) 1500 cubic metre cuts or about 1000t of fresh material weight. As you well know, effluent production or flow is highest at the time of ensiling and for a couple of days following and then declines to almost nothing after two weeks. The regulations assume an extra 6.7 litres per cubic metre for the other areas of the clamp, most likely rainfall on these areas.

 Do clamps in a barn need less effluent storage?

Unfortunately the rules make no allowance for indoor or outdoor clamp space. The calculation is based on effluent produced during the fermentation, not rainfall and the amount of effluent produced isn’t altered by the presence of a roof.

 My high DM silage produces almost no effluent so do I still need a tank?

Yes you do – the storage requirements are the minimum legal requirement and unfortunately the law is the law.

So that’s how big the tank needs to be, but how should a silage effluent tank be constructed?

The regulations are not quite so clear on this element. The rules state that the tank needs to be resistant to effluent and maintenance free for a minimum of 20 years! That’s correct – maintenance FREE for 20 years – in silage effluent. So that’s quite some requirement, no joint sealant manufactures will guarantee 20 years so that means the tank must be one-piece construction. One piece, could that be mass concrete, plastic or fibreglass?

Due to the low pH – high acidity - of the effluent, no cementitious material (including cement) will survive 20 years immersed in neat effluent. I am not sure if anyone is making plastic tanks to the sort of size that is required, but there are lots of options for fibreglass of some sort. So it’s a one piece fibreglass tank – or tanks that’s needed.

Can I just run silage effluent into the slurry or dirty water system?

Unfortunately that’s not an option for a number of reasons. Firstly mixing silage effluent with livestock slurries – urine in particular – produces large volumes of hydrogen sulphide. Not only does this gas stink, it will also kill you in an amazing short time at alarmingly low concentrations! Secondly, as stated above, there is a law instructing you to have a tank of a certain size and construction, so you have to have that tank.

So if you have the correct tank are you all sorted? Almost, again it’s not quite that straight forward. The regulations were designed for livestock farm silage clamps. The sizing rules are the same today as they were in 1991. There were not too many people storing over 10,000 tonnes of maize silage in 1991. What this means is that the regulations might not really be suitable to a large scale AD plant. If you are cutting low DM maize – and you really shouldn’t be – you might soon overwhelm even a very large tank. Having tanks of the required capacity is no defence if you cause a pollution incident. You therefore need to manage and control the effluent levels during the harvest period.

Now for the good news, these rules only apply for new, enlarged or reconstructed clamps, so an old existing clamp does not need this size of tank. However if you pollute – you will be prosecuted whatever size of tanks you have. Secondly, the regulations allow for the diversion of rainfall into the dirty water systems outside of the peak flow periods. This requires a delicate bit of drain plumbing and you need to make sure you can’t accidentally contravene the regulations but it will mean you don’t have to tanker out gallons of rainwater.

If you want to discuss the rules and regulations around silage effluent tanks or any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com