## 3 Case Studies of Pilot and Full-Scale STRBs
### Water Works Sludge (England)  
The handling of settled coagulated sludge from water works (WW) represents a big concern for the European water industry [58]. Generally, this type of sludge presents poor dewatering properties that hinder its easy beneficial reuse. WW sludge is a by-product of freshwater purification processes that has a low solids content, namely river and lake water and/or groundwater purification. Coagulation with, for example, aluminium sulphate, polyaluminium chloride, ferric sulphate, or ferric chloride is applied to remove impurities, resulting in the production of sludge that is usually sticky, not easy to handle, and often has an unpleasant odour. One management option is landfill disposal, but this is not a cost-effective practice [58]. Thus, its onsite dewatering or its transportation to the nearest WWTP is typically preferred. Although mechanical methods are usually applied for WW sludge dewatering, pilot and full-scale STRBs have been tested over the last 10 years.  
WW sludge generally has a low solids content (0.1-0.4% ds), i.e., lower than the solids content in a typical activated sludge (0.3-0.6% ds) from WWTPs. It also has a high iron or aluminium content, due to the upstream coagulation process that usually utilize ferric- or aluminium sulphate/chloride. The organic matter, oil, and fat contents are generally low (\(<\)1000 mg/kg ds) [58; 59].  
Table 3 presents general information on the STRBs in England for WW sludge dewatering. The STRB at the Hanningfield Reservoir in Essex (Northumbrian Water) is located at an approximate distance of 3 km from the WW facility and was built in 2012 with a treatment capacity of 1275 t ds/year (Figure 8a; Table 3). This system is, today, the world's largest full-scale STRB facility, with 16 basins covering an area of 4.5 hectares. Pilot STRB beds (Figure 8b,c) were first tested between 2008 and 2013 before the construction of the full-scale system [53; 60]; six pilot beds of 20 m\({}^{2}\) each were installed to study the dewatering properties of the sludge from the WW. The pilot bed design followed that of a full-scale STRB.  
At the Whitacre WW (Severn Trent Water), sludge was treated in a pilot STRB (Table 3). At that facility, the de-sludging and washing of the pulsator clarifiers produces mineral ferric sludge, since the previous water treatment includes coagulation with ferric sulphate. The ferric sludge was stored in a plastic tank and then loaded onto the pilot STRB over a five-month period in 2015. The pilot STRB setup included three beds of 1 m\({}^{2}\) surface area each.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Water Works Facility** & **Period** & **No. of Beds** & **Total Treatment Area (m\({}^{2}\))** & **Sludge Type** \\ \hline Hanningfield WW (P) & 2008–2013 & 6 & 120 & FS \\ Hanningfield WW (F) & 2012–2014 & 16 & 42,500 & FS \\ Lumley WW (P) & 2010 & 3 & 3 & AS \\ Whitacre WW (P) & 2015–2016 & 3 & 3 & FS \\ \hline \hline \end{tabular}
\end{table}
Table 3: STRBs in England for water works sludge treatment (P = pilot, F = full-scale, FS = ferric sludge, AS = album sludge, WW = Water Works) [45; 53; 60].