## 3 Case Studies of Pilot and Full-Scale STRBs
### Overall Evaluation  
Generally, the use of STRBs for industrial sludge dewatering has been proved to be a technically and economically feasible alternative to mechanical methods, providing a series of benefits as shown in Table 6[1; 61]. The main advantage of STRBs in industrial sludge dewatering is the significantly lower operation and maintenance costs compared to those of conventional technologies. It is reported that industrial sludge management could account for even 50% or more of the overall wastewater treatment costs when using mechanical technologies, mainly due to the huge energy input required [37; 49; 62; 63], which is not the case for STRBs. Current experience from pilot and full-scale STRB systems dictates that sludge from WW can be treated in STRBs, if the contents of organic matter, fats, and oil are not too high. Reeds can be established in these systems, and their growth is not inhibited by this sludge type, e.g., ferric sludge. STRBs for WW sludge demonstrated a good dewatering capacity, as implied by cracks on the residual sludge surface once the water was drained out of the system. The SLR applied during the trials was between 30 and 45 kg ds/m\({}^{2}\)/year, and the ds content of the residual sludge reached 40% in pilot beds. The drained water from the beds showed a maximum infiltration rate of 0.02-0.03 L/m\({}^{2}\)/s after loading, and then declined fast over the following few hours. On the other hand, the dewatering of biological or alum sludge through mechanical processes results in a solids content that does not usually exceed 15-20% [43; 64; 62]. Thermal processes can deliver higher solids contents, but the high energy required for these systems makes them often not financially feasible and also technically challenging [64; 65]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Criterion** & **STRB** & **Mechanical Systems** \\ \hline Land area demand & High & Low \\ Investment cost & Moderate/high & Moderate \\ Operation and maintenance cost & Low & High \\ Power input demand & Minimum & High \\ Use of chemicals, e.g., polymers & No & Typically required \\ Quality of final biosolids & High & Low \\ Life expectancy of main components & 20-30 years & 5-10 years \\ (re-investment interval) & & \\ Need for skilled operators & No & Yes \\ Nuisance (mosquitos and odour) & No/minimum & Moderate/high \\ Downtime due to, for example, failure or repair & None & To be expected/requent \\ Climate change impact & Positive (carbon sink) & Negative \\ Aesthetic appeal & High & Low \\ Biodiversity enhancement & Yes & No \\ Corporate social responsibility value & High & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Overall evaluation of STRB and mechanical methods for industrial sludge dewatering. Figure 13: Pilot STRB at Hanningfield WW: four sludge loads during the same day (blue curve, m\({}^{3}\)/h) and the dewatering profile (red curve, L/s/m\({}^{2}\)) [53]. The high organic solids content in feed industrial sludge leads to lower ds content and anaerobic conditions in the residual sludge layer. Along with organic matter, the contents of fat and oil and heavy metals are the main parameters that should be monitored and tested when sludge of industrial origin is to be treated. It is advised to control the fat and oil content in the feed sludge at the WWTP by enhancing its upstream removal, to reduce the amount that reaches the STRB. Moreover, an STRB trial is suggested before the implementation of a full-scale system in order to evaluate the sludge dewaterability using the STRB technology and optimize the system performance and operation, e.g., in terms of the applied SLR and required number of beds. Since heavy metals are conservative pollutants and cannot be degraded in an STRB system, they accumulate in the residual sludge layer and the porous media. Elevated heavy metal concentrations can be found in some agro-industrial wastewater, such as abattoir effluents, which are usually discharged into a municipal WWTP.