Environmental Engineering: Fundamentals and Design - Chapter 11

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TOC > Ch. 11 > 11.10 Sludge treatment and disposal

# Chapter 11 Wastewater Treatment

James R. Mihelcic, David W. Hand and Martin T. Auer

## Sludge treatment and disposal

### Solids Management

We have removed solids at various points in the wastewater treatment process: primary sludge composed of the sewage solids collected in the primary clarifier and waste activated sludge from the secondary clarifier.

This sludge presents two problems. First, it is rich in organic matter and will consume oxygen and create foul odors if delivered to the environment without further treatment. Second, even though it's called sludge, it is mostly water and is difficult to handle in final disposal.

Thus sludge processing involves two steps:

- digestion - to break down the organic matter
- dewatering - to reduce the water content and make it easily handled

The treated sludge is then sent on for final disposal.

### Digestion

Organic matter is broken down and made inert in a process called anaerobic digestion. Specialized bacteria, able to live without oxygen (anaerobic conditions), use the biodegradable organic matter as a food source, producing carbon dioxide and methane gas and inert organic matter similar to peat moss.

Two-stage digester

### Digestion Process

Anaerobic digestion is a two-step process. In the first step, bacteria and sludge are mixed together in a heated tank called a digester. This is where the biological activity takes place and biodegradable organic matter is broken down producing gases and inert organic matter. The sludge is then delivered to the second step. This digester is not mixed and the sludge separates with gases at the top of the digester, liquid in the middle and solids on the bottom. Gases that have risen to the top are collected and burned. The liquid is returned to the beginning of the plant to be treated again. Solids are sent to the next step for dewatering.

### Dewatering

Left: Belt Press There are several different ways to remove water from sludge. One of these is called a belt filter press. Sludge is placed between two cloth belt filters that pass over a series of drums or rollers squeezing out the liquid. At the end of the press, a knife scrapes the sludge cake off of the belt filter. Liquid removed during pressing is returned to the beginning of the plant for further treatment. The sludge cake is then sent on for final disposal.
Sludge is pumped to the belt press (brown pipe in Belt Press figure) and is delivered at the top where it is spread evenly across cloth belts by black rubber dividers (figure above). | Sludge then passes between cloth belts, squeezing water out which is returned to the beginning of the plant for further treatment. | The dewatered sludge "cake" passes over a knife edge, separating the sludge from the cloth belt and delivering through a passage (blue) to a waiting truck below.

### Disposal

Digested, dewatered sludge can be disposed of by burial in a landfill or by burning (incineration). Landfills which receive sludge must be designed to prevent contamination of nearby surface or groundwater supplies. The gases and particles produced in incineration must be collected as well to protect air quality.

Digested, dewatered sludge is also applied to agricultural land and made available to the public for personal use as a soil conditioner and fertilizer. This disposal option requires that the sludge be treated to reduce (for agricultural use) or eliminate (for public use) pathogens. Sludges treated in this manner are called biosolids and their application is controlled to protect the public health and welfare.

Digested, dewatered sludge in storage prior to land application | Spreading biosolids on fallow fields

#### Credits

Solids Management Flowchart illustration: Marty Auer.
Two Stage Digester photo: Andy Whaley (Michigan Tech).
Wastewater Solids Digestion animation: © 2007 State of Michigan.
Belt Press photo: Andy Whaley.
Top of Belt Press photo: Andy Whaley.
Cloth Belts on Belt Press photo: Andy Whaley.
Belt Press Exit photo: Andy Whaley.
Sludge Storage photo: Andy Whaley.
Sludge Spreading photo: Dr. Jim Ippolito, Colorado State University. Used with permission.

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