Category Archives: Anaerobic Digestion

Anaerobic Digestion

Capital Costs – System Efficiency Is Crucial

Posted on by Elliot Volpe
27
Sep

Capital costs of an Anaerobic Digestion plant can be high. With the need for surveying, licensing, and other assessments before the construction even begins, it can be a costly process.

From designing the system, to installing and commissioning, important calculations and decisions have to be made to determine the system characteristics. Highly skilled people mean high costs.

A study done into the cost of AD site construction was carried out by the International Energy Agency. They have estimated the range of costs per kWe between £3,000-£6,915. With 200 cows, as a rule of thumb, will require a 200m3 digester. A 200m3 digester equates to 20kWe.

On average, it is £5,000 per kWe. This means the cost of a 20kWe plant will be in the region of £100,000. Add on top of that roughly 10-15% for surveys, licensing, and assessments.

Dependent on the pre and post processing of the feedstock, this can increase the price too. As with anything, there are options with varying costs. A system can be made cheaper by using a flexible tank or a semi-permanent lagoon cover. You can remove unnecessary processes within the treatment if they are deemed so.

These costs incurred are purely capital costs (CAPEX). If we think about operating costs (OPEX) then dependent on the system performance, this can determine how quickly the capital costs can be recovered. if you have an inefficient system, then it is clearly going to take far longer for you to pay back the CAPEX costs. That is why system efficiency is absolutely critical. Often, plants need to be running at around 90% efficiency to hit their cost recovery targets.

Anaerobic Digestion

Pipeline Flow – Consistently Inconsistent

Posted on by Elliot Volpe
15
Sep

Sometimes, the flow in a system is predictable and acts exactly how we want and need it to. However, other times it can be irrational and not behave as it should. Because of this there are methods we can introduce to stop units in systems from being overloaded by a sudden influx of solids.

If a tanker is pumping its contents into a system, having just collected from a location, you can be confident that the solids are still in suspension. Therefore, they will be somewhat consistent throughout the liquid, thus meaning it will not overload the system, as long as it is pumped at a suitable speed.

If the same tanker has collected waste from another location, and once the contents have been retrieved the tanker is then left for 24 hours, the solids will start to separate and fall to the bottom of the tanker. After 24 hours has passed and the contents are pumped out, there will be a high solids loading on the system, potentially overloading it.

 

How Can This Be Solved?

One idea can be to introduce a holding/reception tank into the system. This can be specified to the flow rates required or dwell times needed. Doing this will ensure that equipment will not be overloaded straight away. You can use the tank as a storage facility, so when the system is ready, the medium can be pumped out of the tank.

However, this will not solve the problem directly. The use of a holding tank may store the medium, but just like the tanker being left for 24 hours, the holding tank will act exactly the same. If it is not pumped out quick enough, then the heavy solids will settle at the bottom.

To overcome this, much like inside a digester itself, the holding tank would need to be mixed or agitated in order to keep the solids in suspension. This could be through the form of mechanical means such as a propellor mixer. Other methods can be ones like recirculation pumps that are strategically placed adjacent to the tank to ensure the contents are constantly being mixed.

Another aspect to consider is where your pump is located in comparison to the tank. If your suction line is at the bottom of the tank, then you are naturally going to be pumping from the bottom of the tank first.

Anaerobic Digestion

Rising Food Waste Concerns – WRAP Report

Posted on by Elliot Volpe
06
Sep

Food waste continues to be a major problem in the UK, as published in the WRAP food waste report. With so many outlets offering quick, easy and cheap food it often takes priority over using what we have in the cupboards. Or, when it doesn’t seem appealing to throw together some ingredients and make a nutritious meal, instead we opt for the easy, far less healthy option.

If there is one thing the pandemic has taught us (besides not needing to bulk buy toilet rolls) is that when there is threat of food shortages and fast-food outlets are closed, that we can in fact make do perfectly well with the necessities. This is proven by the fact that 79% of UK citizens undertook additional food management behaviours in 2020.

Food waste dropped dramatically in the first lockdown (by 43%) and remained well below pre-pandemic levels across all of 2020. Many people adopted the behaviours such as freezing/defrosting food, using up leftovers and batch cooking.

In the UK, 70% of food waste (post-farm gate) comes from households, equivalent to a value of over £14 billion. This equates to 20 million tonnes of greenhouse gas (GHG) emissions. With food waste levels replicating those seen back in 2018, it does raise concerns that people have shifted out of their pandemic mindsets and are well and truly back into old habits.

 

What Does the WRAP Food Waste Report Mean for AD?

Food waste isn’t a positive thing. If it can be consumed, then it should be. Nobody wants to see food being wasted, especially as most of the time it is perfectly good food. As far as AD is concerned, food waste can be used in the process.

It is certainly better for food waste to be brought to AD plants for processing, meaning that it plays a part in renewable energy production. Otherwise, food waste goes to landfill and becomes yet another pollutant of GHG in the atmosphere.

Yes, the more food waste coming to AD sites to be processed, means more money for the system providers, but a moral approach has to be considered here. Just because it will mean more money in back pockets, that does not mean it should be wholly supported.

If there is valid reason for the food waste, then it should be processed by an AD plant and turned into renewable energy. However, we should be looking at ways of dealing with this problem at its source. That is by educating people on how to change behaviours with regards to food management and waste. But, also educating them on the effects their food waste is having on the environment they live in.  

 

Reference – WRAP Report

Anaerobic Digestion

Anaerobic Digestion: A Plants Location

Posted on by Elliot Volpe
03
Sep

As Anaerobic Digestion became more popular from 2010 onwards, this meant the construction of Anaerobic Digestion plants became more appealing, even on a smaller scale.

Because of this, we now see Anaerobic Digestion plants situated across the UK. As the Feed in tariff started to phase out, we have seen less small-scale construction over the last few years.

However, there are different types of plants that are slowly becoming more viable, allowing to the right licenses, and ensuring that authority guidelines are followed.

The locations of small-scale Anaerobic Digestion plants are commonly on farms, as a way of utilising the waste and turning it into profit. Because of this, these small-scale plants lend themselves to wherever the farms are situated. Although, as part of the planning process, accessibility is one of the main focal points.

For larger scale plants, we often see a more centralised location that acts a “hub”. This centralised plant lends itself to waste collection companies and their profitable work of collecting waste from other people’s homes, often at a cost, and then putting it into their digesters. Then making money from the output of the system. Meaning this set up is a viable and healthy business opportunity.

Locals may opt to bring waste to the location, rather than having it collected. When this is done, they may be charged a gate fee. The idea of this is to allow the plant to receive payment for the disposal of someone else’s waste.

Larger scale Anaerobic Digestion plants cost more to construct and certainly take a lot of time and planning to be fully operational. This is why larger Anaerobic Digestion plants are not typically on farms or in gardens. They are often found on purpose-built sites, as it would be ineffective to construct a multi-million-pound plant on a site that is not fully able to cope with any aspect of the process.

More on Site Locations & Layout

Anaerobic Digestion

pH – It’s All About Balance

Posted on by Elliot Volpe
31
Aug

The pH balance in the anaerobic digestion process is critical. If the pH level is unstable, it can lead to underwhelming biogas yields, and often leads to odour from the process. There is a sweet spot for the pH levels to enable sufficient yield.

In an experiment conducted on grass silage and its biogas yield, dependent on process variables, it showed there is desirable combinations in the systems. These optimum points are largely down to the temperature and pH levels in the digester.

A mesophilic process normally sees the temperature set between 30-38˚c. In this experiment in particular the temperature was set at 35˚c. The pH levels were tested at 3 different levels. These were: 6.5, 6.8 & 7.2

The established, best combination for a mesophilic process was a pH of 6.8 at 35˚c. This combination saw a biogas yield of over 60%.

Multiple pH levels and temperatures were then trialed to find the best overall yield. The highest yield from the experiment as a whole, was achieved from having a temperature of 45˚c, at a pH of 6.5. It is thought that the reason for this is that the pH levels and temperature allowed for rapid growth of the bacteria that produce the biogas.

Continuous monitoring of the pH level in the digester is essential. It can be the difference between producing high amounts of biogas or unsatisfactory amounts. Less gas produced, equates to a less profitable process.

More on pH in the process

 

Reference – Sibiya, Noxolo & MUZENDA, EDISON & Tesfagiorgis, Habtom. (2014). Effect of Temperature and pH on the Anaerobic Digestion of Grass Silage. 

Anaerobic Digestion

How to Achieve 23% More Biogas Yield

Posted on by Elliot Volpe
27
Aug

Biogas yield is one of the main priorities in any Anaerobic digestion process. The more gas your digester can produce, the more gas you can use to generate electricity, or feed back into the grid. In short, more gas produced = more money made.

Sisal fibre was used in an experiment to explore the link between substrate degradation and biogas yield in a process. This was to see how the industry can get the most from their AD system.

The report shows that the sisal fibre degraded in the digester went from 31% to 70% in the experiment. Biogas yield from this particular experiment increased 23% m3 CH4/kg.

How was this done? Maceration.

It has been proven that maceration of the feedstock can lead to quicker degradation of the substrate, in turn, producing a higher gas yield from the process. In this experiment, the original substrate used was raw and untreated. The substrate was then macerated to 2mm to see the difference in process outputs.

Of course, macerating feedstock to 2mm in an AD system may not always be practical. However, this experiment proves the importance of macerating feedstock to get the most out of your process.

More on Biogas Yield

Reference – Performance Improvement for Sisal Waste Anaerobic Biodegradation by Digester Redesign and Feed Size Reduction (scirp.org)

Anaerobic Digestion

What the Temperature Means for Your Process

Posted on by Elliot Volpe
24
Aug

Anaerobic digestion requires a certain process temperature. Heat in the digesters is what makes the breakdown of the feedstock possible. There are two main temperature ranges to break down the feedstock effectively. These are:

  • Mesophilic
  • Thermophilic

Mesophilic temperatures in a digester are usually between 30-38˚c. This is the most common temperature range as it promotes a stable environment for the feedstock to be broken down, with minimal risk of toxicity, and minimal heat losses within the system.

The digesters for a mesophilic process are usually larger as the heat does not need to be intense for the reaction to take place.

One disadvantage to the mesophilic process is that an extra stage of pasteurisation needs to be added to adequately kill off pathogens within the feedstock. Also, due to the lower heat, the process itself takes longer than other methods.

Thermophilic temperatures are commonly upwards of 50˚c. Although this type of process is much more seldom than a mesophilic process, it does have many advantages.

In some cases, a thermophilic reaction is said to be up to ten times faster than a mesophilic process. The mesophilic process can often take 20-30 days at around 35˚c, whereas it potentially could only take 10-12 days at temperatures above 50˚c.

The advantages of a thermophilic system is that hydraulic retention time (HRT) is greatly reduced, meaning that you can get more processed through the digester. Also, you don’t have the need to pasteurise the feedstock as the thermophilic temperature is enough to kill pathogens.

One disadvantage with a thermophilic process is that it can be much more unstable compared to a mesophilic system. Another disadvantage is that it requires a large amount of energy to get up to temperature. With the higher required temperature inside the digesters, it also means there is much more heat loss in a thermophilic process.

Anaerobic Digestion

Treating Your Feedstock Properly

Posted on by Elliot Volpe
20
Aug

What you put into your digester, determines what comes out. In your process, you have to ensure that the feedstock is producing sufficient yield to make it viable.

Feedstock in the process can be anything from household food waste, wastewater, sludge, or even energy crops that can be specifically grown to get the best yield from your digesters.

Often on farms, waste from the animals is used for the process, however, the waste from animals produces far less biogas per ton than particular energy crops. This is contrasted by the fact that the cost, security, and constant production of the waste allows the process to be stable. Whereas, energy crops have to be grown and then harvested, along with many other variables that could affect the process.

For reference, cattle slurry can produce up to 16m3/t of biogas yield. Whereas some energy crops can produce in excess of 600m3/t due to their properties.

Treating the feedstock is imperative in the AD process. You don’t want to have large particles of substrate in your digesters, making the breakdown a prolonged and difficult process.

The best way of ensuring that your digesters are being fed with the correct size substrate is maceration of the feedstock. The homogenisation ensures that every particle is broken down to allow the most effective biogas yield.

Also, by macerating the feedstock, it reduces the strain on the pumping aspects of your process. This means that material can be transferred from a storage vessel, into the digesters efficiently. The same can be said for transportation after the reaction has taken place.

More on Feedstock Treatment

Anaerobic Digestion

The Benefits of Anaerobic Digestion

Posted on by Elliot Volpe
19
Aug

There can be many benefits of Anaerobic digestion. It produces high volumes of energy from renewable sources. The process itself can be time consuming to set up and get right. However, when correct, it is an effective alternative option to fossil fuels.

One of the main benefits of Anaerobic Digestion is that the process slashes green house gas (GHG) emissions. Methane, which is released into the atmosphere by animal, food, and industrial waste, is said to be 23 times more potent than carbon dioxide (CO2). Anaerobic digestion takes potentially hazardous waste and converts it into forms of energy.

The digestate that’s left after the reaction has taken place is rich in nutrients and a great fertiliser to spread on land. The digestate itself will have had its odour reduced by up to 80%, as well as far less pathogens in the digestate after the AD process.

Another of the benefits of Anaerobic Digestion is the financial side. An AD project can give direct financial returns to the process owner or investors.

A CHP unit can be used to allow the site to be self-sufficient, whilst exporting any excess back into the grid. Alternatively, the biogas produced by the process can also be cleaned up, to ensure it is pure methane, and then exported back into the grid. This methane can potentially be used for cooking, fueling cars, trains, and buses.

More on Anaerobic Digestion

Anaerobic Digestion

The 4 Stages of Anaerobic Digestion

Posted on by Elliot Volpe
18
Aug

Anaerobic digestion can sometimes be a lengthy process. The feedstock has to be pumped into the digesters and left for a certain amount of time for the process to take place. It goes through four main stages when in the digester.

  1. Hydrolysis
  2. Acidogenesis
  3. Acetogenesis
  4. Methanogenesis

Hydrolysis is the first step in the breakdown of the feedstock. This stage sees proteins, lipids, and carbohydrates, broken down into smaller, organic molecules. These smaller molecules are amino acids, fatty acids, and sugars.

The second stage of the Anaerobic Digestion is acidogenesis. This is where the organic molecules are then further broken down into basic compounds such as organic acids.

Acetogenesis then takes place and creates acetic acid, hydrogen monohydride (H2), ammonium (NH4), and carbon dioxide (CO2).

Finally, the last stage is methanogenesis. It is at this stage where the bacteria give off the biogas. The biogas created is a mixture of 45-85% methane (CH4) and 15-45% carbon dioxide (CO2), depending on process variables.

More on Gases In the Process