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Sewage Sludge Management

By Rami Elias Kremesti M.Sc., CSci, CEnv, CWEM

 

Introduction

In March 2026, I started working at a large sewage treatment works in East London in the UK and I have been learning more about the science and engineering of sludge management. Nothing has ever felt more like Alchemy than what we do at this mega sewage treatment works… When you see the incoming waste water, the solids, grit and sludge removed from it then the final effluent in which geese and ducks like to bathe, it is very satisfying… Even while working in the stinky press hall, I had poetic experiences… One time i felt like saint George fighting with dark, filthy masses… it was liberating…

 

The process of sewage treatment produces a by product, sludge, which requires careful management… In this page we look at this fascinating aspect of environmental engineering.

The Mathematics

Every 100 litres of typical municipal sewage produces 0.02 to 0.03 kilograms of dry sludge solids. However, because untreated wet sludge is mostly water, this translates to roughly 1 to 2 litres of liquid wet sludge generated per 100 litres of treated wastewater. And the 1 litres or 1 Kgs of sludge has about 20 grams of dry solids. This translates to a sludge that has 1-2% dry solids. Municipal sludge is thickened and dewatered using various technologies.

Types of Sludge

When the raw sewage enters the works, it typically goes through a screening process and after that it is primary settling. The sludge and scum produced here and mixed are called primary sludge which typically has a higher calorific value than secondary sludge because it contains fats, oils and grease. Secondary sludge is the sludge that is wasted in the ASP activated sludge process which is mostly made up of bacteria. Primary sludge burns better after dewatering in the incinerator.

Primary sludge also produces significantly more bio-methane in anaerobic digesters than secondary (waste activated) sludge. Primary sludge yields roughly 50% more methane per unit of volatile solids because it captures raw, easily digestible fats, lipids, and long-chain organics. Secondary sludge consists mostly of microbial biomass that has already been partially oxidized and broken down during aeration.

 

How Is Sludge Pre-Treated and Dewatered

Typically sludge is mixed with a polymer, like a poly acrylamide, which helps with the dewatering. Sludge is normally thickened then dewatered using different technologies. Dose rates are between 6 kg and 15+ kg polymer per ton of dry solids sludge.

Picket Fence Thickeners

Plate Presses

Bucher Presses

Centrifuges

Aqua-Belts or Gravity Belt Thickeners

 

Fouling Mechanisms of Filter Presses:

1. Blinding Mechanisms (Physical Plugging)

Blinding is primarily a mechanical process where the physical openings (pores) between the woven yarn strands become obstructed by sludge solids. It can take different forms:

• Particulate Entrapment (Internal Blinding): Fine suspended particles and colloidal matter bypass the initial cake layer and become wedged deep inside the intersection of the warp and weft yarns. Once trapped, they cannot be easily dislodged by standard cake discharge.
• Fines Migration: Under high compaction pressures, smaller particles are sheared off from the sludge flocs. These “fines” migrate toward the cloth medium, forming a dense, highly resistive boundary layer right at the cloth surface.
• Fiber Stapling: Fibrous materials common in municipal sludge (like hair, cellulose, and synthetic textile fibers) thread themselves directly through the cloth mesh. They act as anchors, catching other particulate matter and initiating a localized blinding zone.

2. Fouling Mechanisms (Biological Deposition)

Fouling involves the structural modification of the cloth surface via sticky organic accumulation, and living biofilms.

• Extracellular Polymeric Substances (EPS): Municipal sludge is rich in EPS—biopolymers secreted by microorganisms containing proteins, polysaccharides, and lipids. EPS acts like a highly viscous “glue” that coats the filter fabric, binding fine particles together into an impermeable, jelly-like glaze.

• Biofouling: Because sewage sludge is biologically active, the filter cloth surface provides a fertile ground for microbial colonization. Over time, bacteria multiply on the damp fabric, creating a persistent biofilm that narrows the effective pore size and resists mechanical washing.

3. Chemical Scaling:

Sludge conditioning sometimes uses chemical additives like polyacrylamide, lime (CaO), ferric chloride (FeCl₃), or alum. Changes in pH and pressure during filtration cause these chemicals to precipitate directly onto the fabric. Common scales include calcium carbonate (CaCO₃), iron hydroxides, and struvite (magnesium ammonium phosphate).

There are ways in the lab to identify the different causes of loss of performance in a filter press. Some of these are:

 

SEM

FTIR: Can differentiate between poly, fats, oils and greases.

LOI

Optical Microscopy

Air Permeability Test

 

Below is a schematic of a filter press.

Post-Treatment

After thickening and dewatering, sludge is either incinerated or anaerobically digested.

When sludge is incinerated , this process produces heat which can be used in a waste heat recovery boiler to generate steam power. The by product from that is ash rich in oxides, phosphates and iron. This is called ISSA. It can be used to make concrete blocks. Phosphate can also be recovered from it through the process of acid bleaching. It can contain low levels of heavy metals which are separated through the process of acid leaching.

Anaerobic digestion on the other hand produces Bio-Methane and a by-product rich in fibres, N and P known as biosolids which can be used as a fertiliser in some countries. Switzerland has a blanket ban on the application of sewage sludge in agricultural fields.

Monitoring the Processes

Every sewage treatment works has to monitor the sewage treatment process as well as all the subsequent processes in sludge management. The sewage treatment process typically monitors incoming and effluent SS, BOD, COD, Ammonia and Phosphate. Of course things like pH, alkalinity and dissolved oxygen are also important process parameters. For example, if there is no alkalinity in the water, nitrification, the process where ammonia is converted to nitrate, will depress the pH to unacceptably low levels.

Sludge management and process monitoring is more complicated: we need to monitor the Dry Solids of the sludge in various stages, the volatile solids as well as the Temperature, pH, ammonia, alkalinity, DS, VS and Volatile Fatty Acids in the anaerobic digester to make sure that it is healthy. The biogas is also analysed for methane, CO2 and H2S content. Incineration is normally done in an environmentally friendly manner which necessitates flue has treatment to remove ash, acids, and any harmful heavy metals. A sophisticated CEMS = Continuous Emissions Monitoring Systems analyses the flue gas to make sure it is within strict permit parameters.

 

Unsung Heroes of Sewage Treatment Works

Some areas in a sewage treatment works can get smelly just like the toilet where you take a shit. The operators, engineers and process control specialists that work at a sewage treatment works are modern day unsung heroes… I have personally met the boys that work in the press hall scraping sludge cake, and it is hard and smelly work… When the ash system gets clogged, the engineers can get covered in ash which has a high pH and can make your skin itch… One cleaning technician has to constantly go around with his vacuum to clean up ash deposits. The lab analyst that analyses the sludge samples is basically baking shit !!! This is nasty to handle and sometimes smelly. You cannot be a spoiled brat doing this kind of work… And a gentle reminder about those that complain about their water bills about the hard work that goes behind the scenes so that water keeps flowing down the tap and the toilet… Surfers Against Sewage, I am an ex surfer, YOU and everyone that takes a shit are the POLLUTER. And polluter pays…

A Bit of Chemistry

Sludge dewatering polymers are typically cationic poly-acrylamides. The polymers used for treatment primary sludge, which has large flocs, are long and sparsely charged. The polymers used to treat the waste activated sludge are typically cross-linked and densely charged because surplus activated sludge has tiny delicate flocs which are best interspersed in a polymer matrix that creates a more easily dewaterable large floc that can resist the shear forces of centrifuges.

Sludge fed to Anaerobic Digesters is sometimes pre-treated using Thermal Hydrolysis, sort of like a pressure cooker treatment step. This breaks down the cells in the sludge and the fats, proteins and carbs which are then more easily digested by the bacteria in the AD. This process requires heat so there is a boiler that produces process steam. This boiler requires a specific chemistry and demin water to run. Also the incinerator boiler, which typically runs at a higher pressure, has a specific chemical regime that needs to be adhered to. As on a power station, boilers require ultra-pure water to run hence there are typically ultra-pure water production plants on site.

In Conclusion

Few jobs are as rewarding as working on a sewage treatment works because these smelly places protect the environment and they produce power, useful ash and biosolids. These are places where engineers and chemists apply their knowledge and skills so they are also training and learning facilities. I hope I have piqued your interest…

 

Rami Elias Kremesti M.Sc., CSci, CEnv, CWEM

 

About the Author

Rami is a chartered water and environmental manager based in the UK. He has been working in the water and sludge management field for over 20 years. His first love was chemistry and he considers himself to be lucky to be able to apply his knowledge and skills in a field of applied chemistry that is so fundamental for protecting people’s and the environment’s health… A civil war survivor, Rami does not comprehend why there are still wars in the world and quotes Hegel in this regard: “The only lesson we learn from history is that we do not learn anything from history”.