Understanding Whisky Production · Unit 8: Distillery Byproducts & Environment

Chapter 18: Distillery Effluent: BOD, COD, DAF, and Anaerobic Digestion

An introduction to distillery wastewater, dissolved air flotation (DAF), anaerobic digestion and trade effluent discharge.

For every litre of malt whisky produced, roughly 10 litres of pot ale are left in the wash still (Edwards et al. 2022, Resources, Conservation and Recycling, 179, 106114). It is a concentrated stream: the same study cites a chemical oxygen demand (COD) of 30 to 50 g/L, and its Biochemical Oxygen Demand (BOD) is high but variable.

Untreated discharge into natural waterways would rapidly strip rivers of dissolved oxygen. Treatment routes vary by site. They can include dissolved air flotation (DAF) to remove suspended solids and anaerobic digestion to produce biogas from organic material.

Sources of Effluent

In a perfect world, distilleries would produce no waste. Everything we bring into the distillery would be converted into things we can sell or recycle. Some materials, such as pot ale and spent grains, can be used as co-products. Wastewater also needs a suitable treatment and discharge route.

Let us first define effluent:

Effluent is defined as the liquid flowing from a process. In the case of a distillery, this is our wastewater.

A simple way to think about effluent is that it is all the water we bring into the distillery, minus the water that goes into the product and the water lost through evaporation. The figure below shows typical water flows through the whisky production process, with the effluent treatment plant shown as bio plant:

Dissolved in this water is the dirt and the chemicals from cleaning, along with small amounts of waste wash, yeast, and lees. The main components of a typical distillery effluent are:

• Waste (wash and lees residues) from fermentation activities
• Raw material or product residues removed from sieves or filters
• CIP rinse water
• Used (dirty) detergent and sanitiser
• Environmental waste from general cleaning
• Product waste from tank rinses

As well as considering what is in effluent, we must also understand what we need to make sure isn’t. This is detailed in the table below:

So, as we now know where effluent comes from and what we need to exclude from it, let us now consider how to measure its key elements.

Effluent and its Measurement

You may have heard this phrase before: “If you don’t measure it, you can’t manage it”. This phrase particularly applies to effluent. Knowing the nature of the effluent allows us to decide how to process, treat, and dispose of it.

We have a few options for ways to deal with our effluent. We will discuss all of them later in more detail. These options are illustrated in the figure below:

Our options for effluent treatment and disposal are affected by the nature of our effluent. Let us look at the parameters we must measure in order to understand the nature of effluent.

Effluent measurement parameters

The following are key effluent parameters that we can measure:

1. Volume (also called hydraulic volume):

The amount of effluent discharged within a set period.

2. Suspended solids:

The amount of suspended solid material or total suspended solids (TSS) in the liquid effluent. This is important because solids are harder to treat, and often require separate treatment steps. We will explore these later.

3. COD:

The chemical oxygen demand (COD) measures the theoretical amount of oxygen required to break down (oxidise) biological matter in the effluent. We can measure COD in several ways. A common test is to use a mixture of sulphuric acid and potassium dichromate to provide the oxygen. The potassium dichromate is used up in the process as it oxidises organic matter. So, measuring the amount of potassium dichromate remaining after the test will tell us how much oxygen is required.

4. BOD:

The biochemical oxygen demand (BOD) measures the amount of oxygen that is required by micro-organisms to digest the most readily available nutrients present in the effluent. The amount of oxygen consumed is measured over a five day period. The test is thus known as BOD 5. This value is usually lower than the COD, and the difference indicates the ease/difficulty of digestion.

The BOD, COD, and TSS are known collectively as the effluent loading. This basically refers to how dirty the water is, or how much treatment it will need before it can be safely discharged to a watercourse.

5. pH:

This is the measure of the acidity or alkalinity of the effluent. Strongly acidic or alkaline effluent can harm a receiving river and can damage sewer pipes and treatment equipment, so pH is one of the parameters a consent or permit can limit.

6. Temperature:

Effluent temperature matters because a discharge can affect the receiving environment: Scottish water law counts the introduction of substances or heat into the water environment as pollution. Applicable limits and treatment requirements depend on the discharge and its authorisation.

In Scotland, a trade effluent discharge to a public sewer generally needs consent or an agreement with the water and sewerage company, which is Scottish Water under section 24 of the Sewerage (Scotland) Act 1968; some discharges also need SEPA authorisation. For direct discharges to the water environment, SEPA explains the permit requirements. The applicable conditions depend on the discharge and receiving environment.

Effluent composition varies with the distillery, the process and the waste stream sampled. A study of pot ale and spent wash from 22 Scotch whisky distilleries (Edwards et al. 2022) describes high oxygen demand and variation in composition. It cites a chemical oxygen demand of 30 to 50 g/L for these co-products; individual measurements can differ, so a site needs its own sampling data.

Having identified the measurements and discharge conditions, we can look at ways to reduce effluent loading.

Reducing Effluent Loading

When working to improve any aspect of our distillery, we should always focus our efforts on the areas or processes that will have the greatest impact. There would be little point in spending time reviewing the amount of soap used in handwashing, whilst leaving a leaking yeast tank for another day. It is therefore important to identify key areas and monitor effluent strength. So how do we monitor effluent and where are the most important opportunities for reductions in loading?

Flow meters measure the flow rates and the volume. Samples can be taken and analysed to establish the TSS, COD, and BOD values. These samples can either be spot samples (taken at a point in time), or composite samples, where small samples are taken every few minutes using an autosampler and combined to provide an average across a period of time. We can also use in-line meters to measure pH and temperature continuously. This is rather than taking random and single point samples to the laboratory, which can lead us to miss a spill, or a pulse of high loadings.

To reduce loadings, identify the waste streams with the greatest contribution to the site’s total load. Concentration alone does not tell the whole story: both the amount of material and the volume of each stream matter. Pot ale, spent lees, steep water and cleaning water should be sampled separately where practical.

Now that we have identified some of the highest contributors to effluent loading, we can now target reductions in the following ways:

• Collecting last runnings from spent grains or draff, which can contain high-suspended solids and a high COD and BOD. We can reuse last runnings collected into a weak wort vessel for use in the next mash.
• Wort cooling water can contribute to high volumes and temperatures. We can recover and reuse it for other purposes.
• Water and chemicals for plant cleaning generally go straight to drain. They can contribute to high volume, pH and COD. However, we can recycle final rinses and its detergents.
• Pot ale has a very high BOD, so care is needed to ensure it does not go to drain but instead into a valuable co-product, such as pot ale syrup. Malt dark grains were once another outlet, but a Scottish Government study reports that the last remaining malt dark grains plant closed in early 2018.

Reducing wastewater volume can be part of managing the site’s effluent. The following are ways in which a distillery can reduce the volume it produces:

• Use CIP final rinse water as pre-rinse
• Limit washdown hose use (use squeegees)
• Use low flow, high pressure rotary CIP heads

Now that we have learned how to measure and minimise effluent, we will now examine how we can treat the effluent we produce.

Effluent Treatment

As the figure below shows, options for dealing with effluent include pre-treatment before a consented sewer discharge and on-site treatment before a permitted discharge to the water environment. Treatment choices and costs depend on the site, its discharge route and the applicable conditions.

The traditional method of treating effluent was via facultative lagoons, or low-rate trickling filters. Facultative lagoons resemble shallow ponds, where microorganisms break down organic material in effluent.

Low-rate trickling filters are large vessels filled with rough stones, over which the effluent trickles. Microbes on these stones digest and remove the BOD from the effluent.

Both of these methods, however, require large amounts of space and it takes a long time for them to treat the wastewater effectively. Today, they are seldom used for industrial effluent treatment, but they are still used at municipal plants. There is, of course, always an exception to every rule. Facultative lagoons are still used in areas with tropical climates. The higher ambient temperatures in these areas enable quicker rates of digestion and more rapid settling.

The treatment of effluent can be broken down into primary, secondary, and tertiary treatments.

• Primary treatment deals with the pH and suspended solids using a series of screens, settling tanks, and clarifiers, which remove the solid and particulate matter as sludge.

• Secondary treatment then deals with the BOD/COD and uses anaerobic or aerobic treatments.

• We can also use a tertiary treatment, which is a further filtration and disinfection step. Treatment performance depends on the process, the effluent and how the plant is operated. Willow lagoons, reed beds and reverse osmosis are examples of additional treatment options.

Whether treated water can be discharged or reused depends on its quality, the intended use and applicable authorisations.

Effluent Treatment - Primary Treatments

Primary treatment involves removing solids and adjusting pH. A balance tank is usually the first piece of equipment used in this process.

Balance tanks are designed to ensure that any subsequent treatment equipment receives an even flow of effluent. They hold large volumes, which means that any spikes in loading and fluctuations in pH are evened out.

After the balance tank, settleable solids can be removed by gravity, using settling tanks or tilted lamella plates. Flocculants or coagulants are often added to effluent and make the solids easier to remove. These coagulants are only effective at particular pHs, so the pH is also adjusted with acid or alkali dosing at this point.

Another common method used is dissolved air flotation (DAF). In a DAF plant, millions of tiny air bubbles stick to the coagulated solids, float to the surface, and are then skimmed off. The clarified, pH-corrected effluent is drawn off beneath the floating layer.

Sludge from screens and pre-clarifiers can be de-watered to reduce its volume. Its composition and applicable waste rules determine how it may be treated, recovered or disposed of. Copper content and any proposed application to land require particular care: distillery by-products from copper stills can carry enough copper to poison susceptible sheep (NADIS, copper poisoning in sheep).

Effluent Treatment - Secondary Treatments

Two main secondary treatments exist for distillery effluent: anaerobic and aerobic. Anaerobic treatment converts organic matter into methane and carbon dioxide, also known as biogas. Aerobic treatment converts organic matter into carbon dioxide and biomass. We can use a combination of the two in order to maximise the treatment’s efficacy.

Anaerobic treatment

Anaerobic treatment means that the breakdown of the organic compounds, such as carbohydrates, proteins, and lipids, is achieved using anaerobic bacteria in the absence of oxygen. The products of the degradation, including sugars, amino acids, and fatty acids, are further converted to biogas. A key benefit of anaerobic digestion is that it forms ample biogas and much less excess sludge than aerobic treatment (Mainardis, Buttazzoni and Goi 2020, Bioengineering, 7(2), 43). The biogas can be used as a fuel for steam raising, or to run a gas engine powering a generator to make electricity.

An upward flow anaerobic sludge blanket (UASB) reactor treats the effluent using an anaerobic process. A blanket of granular sludge, consisting of anaerobic microbes, is suspended in the tank. Effluent flows upwards through the blanket and is digested by these anaerobic microbes. The upward flow of effluent stops the sludge blanket from sinking to the bottom, while the settling action of gravity means that the blanket is not pushed to the top. The granules form by self-immobilisation of the biomass; the process does not require added carrier media or flocculants.

UASBs produce biogas with a high concentration of methane as a by-product of the treatment process. This biogas may be captured for use as an energy source. The effluent flowing from the UASB has a much-reduced BOD. However, depending on the effluent quality requirements, it may need further treatment, e.g. with the activated sludge process. We will explore that now.

Aerobic treatment

The activated sludge system is the most common type of aerobic treatment system. Other types include the oxidation bed, the rotating disc contactor, and the fluid bed reactor. The activated sludge system consists of an aeration basin leading to a sedimentation basin, and then finally a sludge discharge facility. This process takes advantage of aerobic micro-organisms that can digest organic matter in effluent. The sludge in the system is largely biomass (microbial cells). The biomass absorbs the organic matter in the effluent and uses it to grow more cells, hence increasing the amount of sludge.

Part of the organic matter removed is converted into new biomass (sludge), whilst the rest is oxidised to carbon dioxide. It produces a liquid that is relatively free from suspended solids and organic matter.

An activated sludge process usually includes the following items:

• Aeration tank - where air (or oxygen) is injected into the mixed liquor.
• Settling tank (usually referred to as the ‘final clarifier’ or ‘secondary settling tank’) - where biological flocs (the sludge) are allowed to settle out.

Trade Effluent Discharges and Costs

Trade effluent discharged to a public sewer is subject to the relevant consent or agreement. Operators should check the applicable conditions and charges with their water and sewerage company. A treatment charge is not the same thing as a penalty for breaching discharge conditions.

Treatment on-site can reduce the load sent for further treatment, but its costs and benefits depend on the plant, discharge route and local conditions.

Key Takeaways

Treatment and discharge routes must suit the site and its authorisation. Anaerobic digestion is one possible treatment option; its costs and benefits depend on the plant and local conditions.

Sources and review

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About the author

John Fegan · Editor

Hailing from the land of haggis and deep-fried Mars bars, John was always going to find his way to whisky. He has run Uisge Beatha since 2015 and Whiskipedia.com since 2020.