Thermophilic digestion pilot plant at Northern Waste Water Treatment

Published Wednesday, 26 September 2018

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Authors: R Silbernagl
K Sumadraji

PROJECT DESCRIPTION AND BACKGROUND

EThekwini Water and Sanitation has awarded Aquest Colsen, the contract to implement a thermophilic digestion pilot plant at their Northern Wastewater Treatment Works (WWTW). The aim of this project is therefore to assess the feasibility of Thermophilic Digestion (TD) as an advanced sludge stabilization and treatment method, using anaerobic digestion. Anaerobic digestion can be performed at two temperature ranges; mesophilic 30-40⁰C or thermophilic 45-60⁰C. TD is considered an advanced sludge treatment method that improves process performance of mesophilic digestion (MD) by operating at higher temperatures.
This pilot plant, treating municipal wastewater sludge using TD, is the first of its kind in South Africa. Recent pilot studies and conversion of full-scale installations of TD, mainly in the Netherlands, have proven that the process can be operated in a stable manner harvesting the benefits of TD. However, it is important to verify that efficient operability can also be achieved in a South African context, which is being investigated with this pilot plant project, by measuring organic solids reduction, biogas quantity and quality, as well as sludge quality. Aquest Colsen is responsible for all operations, lab work and reporting in meeting the objectives of this project.

PILOT PLANT DYNAMICS

The pilot plant consists of three 6-meter shipping containers and a 20m3 digester. Container 1 contains two sludge feed buffer tanks. Sludge buffer tanks are supplied with primary and waste activated sludge from the primary thickeners and DAF respectively. The mixed sludge is then pumped with automated semi-continuous feeding to a 20m3 continuously mixed digester. The Digestate drains by gravity into container 2 where it is stored in a third buffer tank before being disposed. Container 2 also contains the MCC of the pilot plant, a local SCADA system and an on-line biogas analyser and flow meter. Container 3 houses the on-site laboratory. To ensure optimal mixing a mixing system developed by Colsen is used. The Digestmix® system is made of a draft tube on the outside of the digester with an impeller at the bottom outlet of the tube. This mixing system reduces scum formation. Since the system is external to the digester, repairs or alterations to the system does not require draining of the digester. The digester is heated with electrical heat tracing on the digester vessel installed below the insulation. This allows for stable automated temperature control of the system at 52 °C.

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