Low Cost Faecal Sludge Management by Anaerobic Digestion Co-Disposal

Before discussing low-cost faecal sludge management and the role of Anaerobic Digestion Co-Disposal, let's start by defining faecal sludge (also spelled “fecal”).

What is Faecal Sludge Management?

Thankfully, that's easy. Wikipedia tells us:

Fecal sludge management (FSM) (or faecal sludge management in British English) is the collection, transport, and treatment of fecal sludge from pit latrines, septic tanks or other onsite sanitation systems.

Fecal sludge is a mixture of human excreta, water and solid wastes (e.g. toilet paper or other anal cleansing materials, menstrual hygiene materials) that are disposed of in pits, tanks or vaults of onsite sanitation systems. Fecal sludge that is removed from septic tanks is called septage.

FSM is necessary in densely populated areas where a proportion of the population is not connected to a sewerage network, and the covering and rebuilding of pit latrines is not possible. This is the case in most urban areas in developing countries. But such services are also used in developed countries where sewerage systems are unavailable. via Wikipedia

Don't be confused by the spelling differences, “fecal” and “faecal” are equally valid.

The following “What is Faecal Sludge?” Faecal Sludge Management video explains the meaning of faecal sludge in more detail:

Low-Cost Faecal Sludge Management

There are huge quantities of faecal sludge generated daily around the globe. Its treatment and disposal are a huge and growing problem as the trend towards globalization continues worldwide. However, the use of the anaerobic digestion process for faecal sludge digestion can be the key to a solution for much of the globe's population in warm climates.

All this is explained in the video below:

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For various reasons, you may not be able to watch the above video, or you may simply prefer to read the content of the above video. If so read on for our  Faecal Sludge Management video transcript:

Biogas Combusted in “Cooking Rings”

Hey, Guys! What do you think is special about the flame on this cooking stove?

What is unusual about it is that it's filled with biogas from the anaerobic digestion of sewage sludge.

After reading this article you will be able to discuss the methane potential of faecal sludge, and understand concerns with centralized treatment. And, you'll have seen an overview of the design of buried masonry brick-built digesters.

This module focuses on possible applications of anaerobic digestion of faecal sludge. It does not go into depth on types of technologies, other types of anaerobic treatment, or the fundamental science. That has been covered thoroughly in many other publications.

If you would like to learn more about this topic we would recommend starting with the anaerobic digestion modules in the third week of the Sandec MOOC on municipal solid waste, the Sandec publication on anaerobic digestion, and basic engineering textbooks on wastewater treatment.

There's not been a lot of research or implementation of the anaerobic digestion of faecal sludge compared to other more well-established technologies. So what are the possibilities?

Methane Potential

Let's start with methane potential, the amount of methane produced per gram of COD which will depend on the level of stabilization of the sludge. So what does this mean for faecal sludge?

Well, we cannot say conclusively because there are not a lot of results for the methane potential of different types of faecal sludge.

But intuitively, one would guess that fresh sludge with very short retention times in containment, like many public toilets or restaurants, would have the highest methane potential. It would have a higher methane potential than for example, the sludge that has been sitting in a septic tank for ten years, undergoing digestion.

Methane Potential Plotted

The following plot shows preliminary results from researchers at Cranfield University. It shows the results of biomethane potential assessments, from 11 pit latrines, two public toilets, and ten portable or container-based toilets, compared to primary wastewater sludge.

They seem to indicate that our intuition is correct. But, based on that we also would've predicted the public ablution blocks would've had higher methane potential. Not so according to this data.

Further Research is Needed

This highlights the need for further research and also more results to confirm whether the results are due to factors such as the seed that was used for the test.

This graph shows the variability of methane potential just from the pit latrine samples used in the study. This variability is also not surprising since we know that the composition of faecal sludge is highly variable.

It also illustrates the need for further research if we are to better understand the methane potential of faecal sludge.

So what does this mean for the anaerobic digestion of faecal sludge?

I'm going to make a division of three levels of implementation based on the scale, the degree of centralization, the required management level, and operational skill.

A high level of implementation for all of these categories is centralized. Many digesters of wastewater sludge are located at large centralized wastewater treatment plants.

That's not optimum for many projects, and medium-scale AD plants can work better in our opinion, than large projects. This is particularly the case in many developing nations.

Community (Medium) Scale Faecal Sludge Management

Medium is community scale. Reactors that are not fully optimized for operational parameters and treatment performance, so require less operational input and have a lower capacity.

Household-level Biogas Systems for Faecal Sludge Management

Lower-scale household-level AD systems are seen in rural areas. These are characterized as small, passive systems. They mainly treat manure and food waste with some co-treatment of faecal sludge. There are millions of implementations in India and China. But, these come with mixed results when applied in other areas.

I'm not going to cover household examples in this module as it's not relevant to our focus on dense urban areas.

Large Centralized Anaerobic Digesters for Faecal Sludge Treatment

So what about the possibility of operating large centralized anaerobic digesters for faecal sludge?

Well, it hasn't been done before, but to evaluate the feasibility one of the first questions is; “Will there be enough methane potential?”

The following are some basic considerations for designing an anaerobic digester.

General Considerations for Designing an Anaerobic Digester

First, you know the temperature of the ambient air. Then you have your reactor and you know the temperature you want to operate the reactor at. For example, mesophilic.

You can also then select your solid's retention time based on your treatment objectives.

You should, by the design stage, know your loadings:

• What's gonna be coming into the reactor.
• The total solids concentration and
• the Mass loading of the sludge.

Degradability in Faecal Sludge Management

You also know the degradability of the sludge.

Based on this, then you can calculate the reactor volume and you can also calculate the amount of biogas that will be produced, including the methane concentrations.

You can then evaluate whether or not the methane that's produced is going to be adequate to generate heat. Heat must, for example, always be available to bring the reactor up to the necessary operating temperature.

Need for Data on the Degradability of Each Site's Faecal Sludge

The problem with this though is that for faecal sludge quality we don't, have reliable data for the degradability.

The temperature is critical as it affects the growth of methanogenic bacteria. At lower operating temperatures you need much longer retention times or they cannot grow in the reactor. They will be washed out and you will not get treatment.

Primary wastewater sludge is around 75% to 85% degradable. And secondary sludge, around 70% to 80%.

Degradability is Critically Important for Biogas Plant Viability

If the sludge is not degradable enough there will not be enough methane production to heat the reactor. This is even more of a concern if you want to operate at thermophilic temperatures for pathogen reduction.

Most Efficient Temperatures and SRTs for Digestion

Required temperatures and SRTs for the digestion of faecal sludge are questions that need further faecal sludge management research.

An additional problem that is just as difficult to answer is that anaerobic digestion is easily upset. And as we know, faecal sludge is highly variable.

Q; What are things that can upset a bio-reactor?

A: Toxins, like nickel or zinc, phenolic compounds, or even high concentrations of free ammonia. lack of pH control, a build-up of volatile fatty acids, fats oils and grease, and shock-loadings resulting in a rapid change of the digester contents. The outcome of that can be a slowdown in biogas production.

If a large-scale centralized reactor is upset, the loss of biogas production for a period of time, it's a management nightmare!

Potentially what all of this means for the anaerobic digestion of faecal sludge at a centralized scale is that it could only be applied in conjunction with co-digestion. In other words, performing the AD process while also digesting the sludge with other organic waste streams that are more degradable.

Only adding small concentrations of fecal sludge relative to the other organics will dampen the variability of faecal sludge going into a reactor.

Examples are brewery wastes, spent grain, market waste, or even wastewater sludge (known as sewage sludge).

Anaerobic Digestion Co-Disposal Community – Scale Digester Advantages

Community-scale digesters are easier to operate because they are not optimized for space, temperature, gas production, or treatment capacity. Nevertheless, they provide an option that is potentially better than septic tanks or other types of anaerobic treatment. That's due to the fact that they are designed to capture biogas for resource recovery and reduce greenhouse emissions.

CDD and BORDA have implemented community-scale systems in India, Nepal, and Tanzania.

There are not yet enough operational results for detailed design recommendations. Digesters of this type have been sized, up to a maximum of a 6-metre diameter, based on solids concentrations collection capacity and frequency of emptying.

Solids and hydraulic retention times are requirements for effluent and solids treatments.

Case study: The Anaerobic Digester Built and Operated by BORDA in Dar es Salaam, Tanzania

This is the schematic for an anaerobic digester built and operated by BORDA in the Mlalakua area of Dar es Salaam, Tanzania.

The capacity of the Faecal Sludge Management plant was based on estimations from operating a similar reactor in the Kigamboni neighborhood. But, with the collection and transport of faecal sludge for the community, as shown here.

The size of the digester was also based on:

• the available land area and the desired complexity of operation so that it could operate at a community scale.
• The resulting design capacity is five cubic meters a day or what is thought to be twenty thousand population equivalents.

An image in the video shows the access road.

First, there is 30 square meters for receiving station for the trucks to discharge, then there is another five square meters for the screening of the sludge.

The dome is constructed to be 50 cubic metres with an expansion chamber of another 21 cubic meters.

Anaerobic Filter

The liquid from the reactor then goes to an anaerobic filter. The sludge goes from there to drying beds which take up another 55 square meters.

And then, the liquid from the drying beds and the anaerobic filters goes here to a banana plantation. which takes up another 170 square meters.

Here (above) are some pictures of what this form of Faecal Sludge Management looks like.

The gate coming into the facility. The receiving bay. Bar screen cover up the dome and expansion chamber.

Faecal Sludge Management – A Summary

In summary, the benefits of using the anaerobic digestion process for Faecal Sludge Management, include:

• reduced sludge and required footprint compared to aerobic treatment,
• reduces methane and sludge for resource recovery, and it can destroy pathogens depending on the operating conditions.

And also, Faecal Sludge Management drawbacks include that the process can be easily upset.

It operates best under controlled and consistent conditions and it requires knowledge and skill for operation. In this module, we learned about the methane potential of faecal sludge, and the design and operating concerns with centralized and decentralized treatment.

For more information, please refer to the CDD and BORDA websites and this biogas curriculum. There you will find a tool for trainers to train masons and technicians on the complexities of the gas-tight construction of digesters.

Faecal Sludge Management videos can be found here, on Youtube

Thanks for visiting. See you next time!

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[First published 6 June 2019. Updated 19 November 2022.]