Sludge Digestion

Anaerobic digestion is the favoured method of stabilising sewage sludge within the UK.  The first sewage sludge digester was built in Exeter at the turn of the 20th Century.  The primary reason for its construction was to reduce the nuisance odours released from the raw sludge; the biogas produced was used to run power the streets lamps throughout the city. 

Marches Biogas is able to offer a range of services focusing on redesigning existing assets to improve the reliability, efficiency and lifetime of a plant.

Our expertise is based on more than 25 years of working on the design, construction and commissioning of sludge digesters. 
Marches Biogas services include:

Consultancy & Troubleshooting;
Risk Assessment & Process Control;
Marches Biogas is able to carry out detailed risk assessment of sludge digesters, particularly with respect to the gas handling and safety systems.
We place considerable emphasis on Risk Assessment and on Hazard Analysis and Critical Control Points (HACCP). These are two cornerstones of today’s process engineering.
The seven steps of HACCP are to:
Conduct a hazard analysis;
Determine critical control points;
Establish critical limits;
Establish monitoring procedures;
Establish corrective actions;
Establish verification procedures;
Establish documentation.

Many existing sludge digesters give unnecessary problems which arise because of inadequacies in the process control system. Marches Biogas has a reputation for being able to analyse the control requirements from the point of view of the plant operator, and then to redesign the process control specification to make plant operation simpler and safer.

Maintenance;
Lithium Trace Testing;

With the emergence of new HACCP (Hazard Analysis and Critical Control Points) regulations, there is a need to verify that the performance of a sludge digester meets the agreed level of treatment for the appropriate method of land application of sludge.
The use of lithium chloride as a trace chemical is considered to be the best method for evaluating digester mixing.
Lithium chloride is introduced to the digester and up to 50 samples of digester output are measured on the first day over a period of 8 hours, and a further 50 samples over the following 4 weeks.
Analysis of the samples leads to the evaluation of 3 parameters: the digester mixing time, the digester hydraulic retention time, and most importantly the effective digester volume (or how much "dead space" there is).

Sludge and Gas Analysis for Effective Biogas Utilisation. 
The simplest way to utilise the biogas produced from sewage sludge is to burn it within boilers to provide the digester with process heat.  This is however, not always the most economical way of utilising the biogas.  Combined heat and power (CHP) units and micro-turbines can be used to produce electrical power and heat.  These forms of energy can be used on site, offsetting the requirement to purchase electricity from the grid, while also claiming Rewnewable Obligation Certificates (ROCs) which are currently worth 4p per kWhr. 

Biogas is produced naturally from the anaerobic digestion of organic matter. The composition of the biogas depends on the characteristics of the feedstock, and in this case is about 60% methane (CH4) and 40% carbon dioxide (CO2) with traces of hydrogen sulphide (H2S).  Biogas produced from sewage sludge will often contain siloxanes. 
Siloxanes are a family of man made organo - silicon compounds containing silicon, oxygen and methyl groups. They are frequently used in the manufacturing of pharmaceutical, and industrial products, of which the residues end up in Waste water Treatment Works. In anaerobic digesters low molecular weight siloxanes volatilise into the gas and therefore siloxanes frequently found in landfill gas and sewage sludge derived biogas include Hexamethycycotrisiloxane (D3), Octamethycyclotrasiloxane (D4) and Decamethylcycopentasiloxane (D5). Siloxanes do not affect the anaerobic digestion process or the digester itself, however, during the combustion of the biogas they are converted into silicates, micro-crystalline quartz and silicon dioxide (SiO2). These can become deposited on the engine abrading the surfaces, and may eventually cause damage to the engine (Schweigkofler et al., 2001).
Marches Biogas can offer a CHP feasibility study which would include an 8 week study of the gas production from the given plant to assess the quantity of biogas produced and the quality of the gas in terms of the methane content, hydrogen sulphide and siloxanes contamination.  A final report will give recommendations on the size of the engine required to match the biogas production, and any gas scrubbing needed to ensure long term health of the engine. 

Marches Biogas specialise in the design and fabrication of:
Foam Traps
There is an increasing requirement to feed digesters at higher rates and with a higher proportion of secondary sludge compared to primary sludge. This has in many cases led to an increase in the number of foaming incidents, which can cause major operational difficulties.
Marches Biogas have developed their own design of stainless steel foam trap which incorporates foam detection and automatic water sprays, which physically break up foam as it is produced.
If foam is not controlled in this way then it can block gas pipework and safety devices, with potentially catastrophic consequences.