Proceedings

The grit problem: contamination of AD feed materials

O’Loan, R., CDEnviro, UK

(free)

Abstract: 
Within the anaerobic digestion and waste water industry the persistent problems associated with contamination with grit, and its detrimental effects, have troubled operators for years. This document details the problems caused by grit and highlights a solution through a case study in partnership with Northumbrian Water. By the inclusion of front-end grit removal systems, the majority of this grit can be intercepted and the detrimental effects of excess wear, sedimentation and efficiency drops can be avoided.

The Grit Problem
Grit accumulates in all stages of the process and brings with it both acute and chronic effects. Short
term issues include screen blinding, reduced throughput and pipe and pump blockages. The long-term
effects are even more problematic and include excessive wear, reduced capacity in the digester and
the resulting cleanout costs, not to mention the need for additional resources for sludge redirection.
These problems have almost become tolerated in AD plants and treatment works today, with operators
becoming comfortable with a duty/standby approach being adopted. This however is not addressing
the core issue; fibre and grit should be removed as quickly as possible to ensure the most efficient flow
process.

The Grit Solution: The Northumbrian Water Research Project:
Bran Sands, located in Middlesbrough, is one of the largest waste water treatment sites in Northumbrian
Water. The site brings in sludge from satellite sites alongside an indigenous feed. The sludge is used
to produce biogas and the site has a total capacity of 4.7 MW. Over 40,000 tonnes of dewatered sludge
is produced every year.

In August 2016 CDEnviro installed equipment at the Bran Sands site with the aim of removing the fibre
and grit in the imported sludge coming to the facility. The goal was to show how this could affect the
changes to digester tank cleanout intervals. The site has three digestion tanks which require a clean
out every seven years due to a build-up of grit, ultimately reducing the biogas generating capacity.
For the sake of a controlled study only imported sludge was used in the project which amounted to
9000m3 over a five-week period. This accounts for approximately 30% of the incoming sludge.
This sludge was screened at the satellite sites and transported to Bran Sands where it was pumped
through the installed equipment. The fibre and grit was collected in different skips which was measured
to calculate the resultant weight. Screened sludge then proceeded through the normal path and into the
anaerobic digestion process.

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