Digesters
Introduction The Process Anaerobic Digestion
Arcata's Sludge Treatment Links and References

Introduction

Digesters are used to stabilize the solids that are removed from the wastewater during treatment. The goal is to reduce the mass of the sludge and convert it into a non-hazardous form so that it may be handled or used with minimal health hazards (Davis and Masten, 2004). This stabilization can be performed by using aerobic digestion, which involves injecting oxygen into the sludge in an open tank, or anaerobic digestion, which takes place in an airtight container. The Arcata Wastewater Treatment Plant uses anaerobic digestion to treat their primary sludge. The Arcata Wastewater Treatment Plant has two digesters, but larger cities, like Los Angeles, have 20 to 30 digesters (Couch, 2008).

What is Sludge?

Digesters are used to stabilize sludge, but what is sludge?

Sludge coming out of a digester

Figure 1: Sludge coming out of a digester (Poppendieck, 2008)

Sludge (Figure 1) is a combination of materials that settle out from raw wastewater and solids that are created during the wastewater treatment process. Sludge can be as much as 97% water, so one of the goals of the sludge treatment process is to separate the water from the solid residues. The separated water is then returned to the beginning of the wastewater treatment process for processing (Davis and Masten, 2004).

How Arcata Deals with its Sludge

Figure 2 displays the path that sludge takes as it moves through the treatment process at the Arcata Wastewater Treatment Plant.

Layout of Sludge Treatment at the Arcata Wastewater Treatment Plant

Figure 2: Layout of sludge treatment at the Arcata Wastewater Treatment Plant (Humboldt, 2008)

The clarifier separates the sludge from the wastewater. Primary sludge, which is usually about 3-8% solids, is pumped from the bottom of the clarifier, through a series of pipes and eventually into the primary digester. Primary sludge is about 70% organic and rapidly becomes anaerobic and foul-smelling (Davis and Masten, 2004). While the sludge is in the pipes, the percent solids in the sludge is measured. The amount of sludge that will be pumped into the primary digester depends on the percent solids in the sludge, and can be adjusted accordingly (Couch, 2008). Sludge is fed into and out of the primary digester continuously (Davis and Masten, 2004). The process of sludge treatment varies if the wastewater treatment plant uses activated sludge for secondary treatment.