Digestate dewatering: the key to AD cost-efficiency?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            

Digestate dewatering is the process of removing excess water from digestate, essentially concentrating the solid components of the digestate which, in turn, makes it easier to transport, store, and utilise as a fertiliser, while minimising the volume of liquid waste that needs to be managed.

And, whilst the initial investment in dewatering equipment can be significant, the potential savings from reduced transportation, improved nutrient utilisation, and increased market value can make digestate dewatering a cost-effective practice for many AD owners and operators.

Here, PWCL’s AD Technical Support Specialist, Ross Henderson, explores digestate dewatering and the significant impact it can make on the cost-efficiency of an anaerobic digestion plant.

The anaerobic digestion process

The Anaerobic digestion (AD) process involves feeding a source of organic materials with a high percentage of moisture content into a vessel without any air getting to it. This process produces biogas and digestate as its output.

In the process of anaerobic digestion, along with biogas, large amounts of digestate are generated which is the left over material after the feed material has been digested in the anaerobic digester.

This digestate is usually separated into solid and liquid fractions. The solid fraction contains most of the phosphate and some potassium and ammonium content. The bulk of the ammonia from the anaerobic digestion process remains in the liquid fraction of the digestate.

Liquid fraction digestate

The main barrier to using the liquid fraction digestate to recirculate back into the AD plant is the build-up of inhibiting biochemicals such as ammonium and chloride, neither of which, are removed in the solid fraction digestate and would build-up to inhibitory levels if recirculated without any further treatment.

Spreading restrictions

What’s more, the high ammonia content of the liquid fraction digestate is the reason why there are restrictions on the type of land that it can be spread on, and time of year when the liquid digestate can be spread called the “spreading window”.

This is to control the risk that digestate will run-off of the land into waterways and cause ammonia pollution.

To manage the restriction of spreading, requires liquid digestate to be stored during the close of the spreading window.

This period is over 6 months and requires anaerobic digestion plants to have large lagoons, which are covered and recently require air abatement to control emissions, to store the liquid digestate until the spreading window opens.

The aim of dewatering

The aim of dewatering the liquid fraction of digestate is to remove the suspended solids and concentrate the dissolved solids, so that the volume of liquid that needs to be stored is significantly less than non-dewatered liquid fraction digestate and removes the need for odour abatement.

Approaches to dewatering

There are 2 main approaches for dewatering anaerobic digestate:

1. Aerobic sludge bed reactor

2. Ultrafiltration/nanofiltration

1. The aerobic sludge bed reactor involves maintaining a population of aerobic bacteria to consume the nutrients within the liquid fraction digestate. This lowers the ammonium and chlorides and is taken up in the biomass of the sludge bed. The effluent from the sludge bed is then further dewatered by laminar tanks or presses usually with the aid of coagulants and flocculant chemicals.

The sludge from this process is a good soil conditioner and the water can be recirculated into the AD plant or discharged.

2. The ultrafiltration/nanofiltration plant uses ultrafiltration membranes to filter out the suspended solids, the second stage ultrafilter separates out the biological oxygen demand and the nanofilter separates the salts, ammonium and chlorides.

The concentrates from this process need to be further dewatered using a filter press or vacuum filter bed to produce a stackable solid, and the treated water is suitable for recirculation back into the AD plant or discharge offsite.

Both approaches can significantly reduce the volume of liquid fraction digestate that needs to be stored until the spreading window is open.

The benefits of digestate dewatering

Reduced transportation costs: by removing excess water, the volume of digestate to transport is significantly reduced, contributing to lower transportation costs.

Improved nutrient recovery: digestate dewatering also allows for better separation of nutrients in the digestate, enabling targeted application based on crop needs, whilst maximising the value of the product.

Higher market value: dewatered digestate can also be sold at a higher price compared to liquid digestate due to its concentrated nutrient content and easier handling, management and transportation.

Reducing the upfront costs of a new plant

The recent advances in membrane filter production technology have reduced the cost and improved the efficiency of ultrafiltration and nanofiltration membranes.

This is significant as it allows them to operate at lower pressures and, ultimately, reduce both the upfront costs and running costs of a new plant.

So much so, in fact, that – with current membrane filter production technology - for the very first time, a feasible plant that can dewater up to 50,000 cubic meters per annum costs less than £1m.

Crucially, this has brought membrane dewatering, as a method of managing liquid fraction digestate, into reach for many anaerobic digestion plant operators.