The application of Lean/ Six Sigma principles to improve process quality at Ireland’s largest Biosolids treatment facility

McCausland, C. O’Connor, F and Croysdill, S. CAW, Ireland



CAW operates Ringsend WwTW in Dublin on behalf of Irish Water. The facility treats for a PE of
approximately 1.7 million. Biosolids treatment at Ringsend consists of a 120 tDS/day CAMBI thermal
hydrolysis, anaerobic digestion, a 4MW CHP, and three thermal dryers. This paper describes how CAW
has adopted a Lean/Six Sigma approach to process measurement and analytics. This approach has
led to data-driven process improvement which has reduced variation in the outputs of the Biosolids
plant, and at the same time increased plant throughput while reducing consumption of key commodities
such as gas and polymer. Three case studies are presented. Case Study 1 describes process control
changes which increased post-CAMBI digestion gas production (m3 /tDS hydrolysed) by 12%. Case
Study 2 describes a 26% reduction in specific gas consumption (m3 /tDS) in the Thermal Drying plant,
and finally Case Study 3 outlines the application of Lean/Six Sigma methods to reduce pre-CAMBI
dewatering polymer consumption by >33% (kg/tDS hydrolysed).


Wastewater treatment plant operators face a number of significant challenges in the current climate.
Across the globe we see the same drivers are dictating the changing face of wastewater treatment.
Tightening legislation continues to reduce the maximum permissible levels of priority pollutants in
discharged effluent and land-applied Biosolids – while also reducing the disposal and reuse routes
available for treated Biosolids. This is against a background of unstable commodity prices – energy,
water and chemicals – in an increasingly competitive market.

Wastewater treatment in Ireland faces all of the challenges listed above. Additionally, the establishment
of Irish Water in 2014 has brought an even greater focus on improving quality and minimising waste in
the sector in Ireland. The drivers listed above incentivise adaptable operators to strive toward attaining
the hygienic and environmental goals of municipal wastewater systems while reducing waste in the
process. Fortunately, many waste minimisation measures in this sector also have positive
environmental impacts, such as carbon footprint reduction and nutrient recycling. Increasingly
wastewater and sludge/Biosolids are seen more as valuable resources rather than waste materials, and
the objectives of modern wastewater treatment processes combine realisation of the maximum
recyclable value of these assets with meeting the discharge consent limits. Whether through biogas
production and use, nutrient recovery and reuse in agriculture, or even final effluent recycling, the
modern wastewater treatment facility has become a complex many faceted production centre.

Biosolids treatment facilities can consist of many different complex processes. Control of these
processes present a significant challenge as the most significant input to the process, the raw sewage,
is highly variable with significant characteristic variation associated with temperature, concentration,
and physio-chemical constituent makeup. This variability can often lead to the process being considered
technically ‘out of the control’ during periods of extended high rain fall or low rain fall, and other issues
in wider network. Variations in raw sewage concentrations received by the wastewater treatment
process, and resulting loading of the works, be it under-loading, overloading or shock loading will have
a dramatic impact on the process, an impact that which would be unacceptable in a typical
manufacturing plant where the operator would insist on controlling such major inputs as the raw material
being received by the process. The result of this inability to control such a significant input to the
process leads to inefficiencies and deviations from expected design outputs and challenges in
maintaining to quality standards and expected design performance with as little variation as possible.
The tendency to inefficiency will also increase with the age of the assets involved. At Ringsend WwTW,
higher than expected loading (above design) to the wastewater treatment processes has posed a very
specific challenge to maintain efficient operation within design boundaries, requiring adaptive thinking
to reduce variability in plant operational processes.

Six Sigma is a rigorous, focused and highly effective implementation of proven quality principles and
techniques, which is often combined with methods from lean engineering as Lean/Six Sigma. These
tools are not new, and have been widely and successfully utilised across the manufacturing and
services industries, with Lean most famously associated with Toyota and Six Sigma with Motorola.
Six Sigma often calls for a paradigm shift in a company’s culture, especially with regard to what are
deemed acceptable levels of defects and waste. Sigma, σ, is a letter in the Greek alphabet which
statisticians use as a measure of variability in a process. The higher the sigma level, the lower the
process variation (or number of defects). A Six Sigma process sets a standard of 3.4 defects/problemsper-million opportunities (Pyzdek, 2014). To try and achieve this level of process performance
companies which adopt Six Sigma culture draw upon a performance improvement model known as
Define-Measure-Analyse-Improve-Control or DMAIC.

Lean/Six Sigma is potentially a very powerful set of tools to aid wastewater operators to reduce
unpredictable variability in costs of operation and improve process quality. In many ways a wastewater
treatment plant is analogous to a manufacturing plant. There are inputs – energy, labour, and
wastewater – which are processed into outputs – treated effluent, Biosolids, and biogas. For each
process stream in the wastewater treatment plant there is a potential achievable quality which is the
known maximum possible added value per unit of input. There is also the actual quality, which is the
current added value per unit of input. The difference between these is waste. Six Sigma methods aim
to eliminate this waste.

CAW operates Ringsend WwTW in Dublin which treats for a PE of approximately 1.7 million. The works
combines secondary treatment (plus UV disinfection) with an advanced sludge processing facility and
Combined Heat and Power Plant. Secondary treatment is achieved through 24 SBRs, and sludge
treatment consists of CAMBI thermal hydrolysis, anaerobic digestion and thermal drying. Operationally
the works is subdivided into 3 ‘streams’; the water stream consisting of the secondary wastewater
treatment works (plus UV disinfection), the sludge stream consisting of a 120 tonne/day hydrolysis plant,
digesters and CHP, and the dryers stream. The following 3 case studies describe how process
variability was reduced and quality improved in 3 process streams at Ringsend where certain critical
variables were known to have deviated outside the boundaries of expected design value ranges.

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