Understanding Whisky Production · Unit 8: Distillery Byproducts & Environment

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

Characterising wastewater streams, dissolved air flotation (DAF), anaerobic digestion biogas production, and Mogden formula charging.

For every litre of pure alcohol produced in a distillery, roughly a dozen litres of wastewater are left behind. The most concentrated stream, pot ale from the wash still, carries an organic load so dense that its Biochemical Oxygen Demand (BOD) is hundreds of times greater than domestic sewage.

Untreated discharge into natural waterways would rapidly strip rivers of dissolved oxygen. Modern distilleries manage effluent through multi-stage on-site treatment: Dissolved Air Flotation (DAF) removes suspended solids, and anaerobic digesters use specialized bacterial consortia to convert high-strength organic waste into renewable methane biogas that directly offsets boiler fuel demand.

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. We are, in fact, not far off; pot ale, grape stems or spent yeast lees are sold on as co-products, and much of our packaging can be reused and recycled. Of the waste we do produce, effluent is the largest by weight.

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 biological 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. To be suitable for releasing into rivers, it must be close to neutral (pH 7). Furthermore, a low or high pH can damage both sewer pipes and receiving equipment, so limits must be set.

6. Temperature:

This is the temperature of the effluent. It should typically be < 20°C if it is to be released into watercourses. High temperature effluent must be blended and/or chilled to achieve a decrease in temperature. High temperatures can also crack sewer pipes, so a maximum temperature must be specified.

It we opt to discharge our effluent to the sewer for treatment at the local authority’s municipal treatment plant, we must first obtain a consent to discharge. Local authorities typically impose limits on the volume and nature of effluent they will accept.

A typical set of consent limits is shown below:

• Maximum volume: 10,000 L per 24 h
• Maximum suspended solids: 500 mg/L
• COD: 800 kg per 24 h period
• PH range: 6-10
• Maximum temperature: 40°C

Distilleries that exceed these consent limits are at risk of legal action, and even of having their sewer sealed up. So, what does a typical distillery effluent look like before it has been treated? The following table shows typical distilling effluent values:

Now that we know what we need to measure, as well as the typical effluent levels produced and accepted at the treatment plant, let us now look at ways to reduce the 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 (typically 24 hours). 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 help reduce loadings, we need to know which elements of liquid waste from production have the highest loading. The table below shows the loading levels in different types of distilling waste:

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 dark grains or pot ale syrup.

Effluent treatment charges are calculated per unit of volume, so as well as minimising the loading of our effluent, it is also important to keep the volume produced as low as possible. The following are ways in which we can reduce the volume of effluent we produce:

• 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, there are several options available for dealing with effluent. Many distilleries discharge their raw (untreated) effluent directly to sewer. Some plants run minimal pre-treatments, such as temperature and pH corrections, before discharge. This pre-treatment lowers the total effluent costs. Other plants have a complete effluent treatment plant on their premises, which comprehensively treats and corrects all effluent to within the required legal limits, before discharging it into a watercourse.

The traditional method of treating effluent was via facultative lagoons, or low-rate trickling filters. Facultative lagoons resemble vast, shallow ponds, where microbes digest the BOD in the effluent. A facultative lagoon is

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.

Primary treatment typically removes 40-50% of the loading, whilst primary and secondary treatment removes 85-95%.

• We can also use a tertiary treatment, which is a further filtration and disinfection step. This step, in combination with primary and secondary treatments, removes almost 100% of loading. Willow lagoons, reed beds, and reverse osmosis are all types of tertiary treatment.

After tertiary treatment, the water can be discharged to a watercourse or be recycled back into the distillery. Even though this water is completely safe, it is unlikely that we will reuse it as product water, as people generally do not like the thought of consuming spirits that has been made from waste.

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 and are generally designed to hold around 12 hours of effluent. This 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 (as ). 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 resulting clarified and pH corrected effluent is known as the supernatant.

What happens to the sludge (the precipitate) from the screens and pre-clarifiers now? This sludge can be further de-watered, reducing its volume. It is then either incinerated as biomass, sent to landfill, or is sometimes used as agricultural fertiliser. This sludge can be further de-watered, reducing its volume even more. It is then either incinerated or sent to landfill. It is often added to the sludge produced from any aerobic/anaerobic treatments. This is an important point for distillers using copper plant, as this sludge will be high in copper. The sludge is typically spread onto land and there are safe guards in place to ensure that any one piece of land is not over dosed with copper. This copper can be ingested by grazing animals, particularly sheep, which can then die from copper poisoning.

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 minimal biomass. 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. Flocculants are added to help hold the blanket together.

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.

Typically, 40-80% of dissolved organic material is converted into organic sludge, whilst the remainder of the BOD is released as 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.

These are

Calculating Effluent Charges

If we do not treat the effluent to the point where we can discharge it to a watercourse, we must pay to have it treated at a commercial or municipal works. The amount we pay is calculated on the basis of the volume and the loading of the effluent.

To understand how the fee is calculated, we will use the system used in the UK as an example. This is known as the Mogden formula:

P = C + V + (St / Ss) S + (Ot / Os) O

You do not need to remember how the formula is written. However, it is useful to know how the components contribute to effluent charges.

Where:

P = cost per m 3

C = conveyance charge per m 3

V = volume charge per m 3

S = suspended solids per m 3

O = COD charge per m 3

St = suspended solids content of trade waste (your waste) in mg/L

Ss = suspended solids content of sewage (of the current waste at the treatment works) in mg/L

Ot = COD of trade waste in mg/L

Os = COD of sewage in mg/L

It is useful (particularly for a start-up business) to ascertain the charge that can be levied onto the business for the expected effluent discharge.

If you want to check what your distillery will pay, you can use this calculator:

Http://www.wrap.org.uk/sites/files/wrap/Mogden%20Formula%20Tool%20March%202015.xls

You will, of course, first need to have a representative sample of your effluent analysed.

Based on this figure, the business can decide whether to install its own effluent treatment plant on site and recoup the capital costs over a period (the payback period). Larger companies tend to have treatment on site because it is more cost effective, but also because they produce more effluent than the local municipal works can cope with.

Key Takeaways

Environmental compliance and municipal discharge charges (such as the Mogden formula) make effective wastewater treatment and anaerobic bio-digestion integral to modern distillery economic viability.

About the author

John Fegan · Editor

John has run Uisge Beatha since 2015 and Whiskipedia.com since 2020. He owns whisky casks and writes from direct experience of their storage, insurance and bottling decisions.

• johnfegan.org