## 3 Case Studies of Pilot and Full-Scale STRBs
### Sewage Sludge with High Chromium Content (Greece)  
A large research project consisted of thirteen pilot STRBs that were built, operated, and monitored in Xanthi (northern Greece) for three years, treating surplus activated sludge (SAS) from a nearby WWTP in Komodini. Three of these pilot beds (Figure 7) received SAS spiked with chromium (Cr) to simulate the Cr content found in sludge produced during tannery wastewater treatment. Each pilot bed was a cylindrical plastic tank (height, 1.5 m; surface, 0.57 m\({}^{2}\)) [29, 55] and contained three porous medium layers (from bottom to top): a 15 cm cobble drainage layer (d\({}_{50}\) = 90 mm), 15 cm medium gravel layer (d\({}_{50}\) = 24.4 mm), and 15 cm fine gravel layer (d\({}_{50}\) = 6 mm). The cobble layer contained drainage pipes connected to the atmosphere through vertical polyvinyl chloride (PVC) pipes open at  
Figure 5: Yellow reeds and poor coverage in an STRB (Stenlille, Denmark) loaded with sludge rich in nickel (courtesy of Orbicon). Figure 6: Nickel concentration (mg Ni/kg ds) in the feed sludge at an STRB in Stenlille (Denmark) [53]. The red line represents the Danish national limit for Ni in sludge recycled in agriculture (30 mg Ni/kg ds). the top. The gravel was obtained from a nearby riverbed. The pilot beds were planted with indigenous common needs (_Phragmites australis_). The three pilot beds (A, B, and C) received three different SLRs, i.e., 75, 60, and 30 kg ds/m\({}^{2}\)/year, respectively [30]. The feeding cycle consisted of one week of loading followed by 3 weeks (wintertime) or 1-2 weeks of resting (summertime). The SAS had an average ds content of 3.1% ds and volatile solids (VS) of 73.7% ds after thickening [56]. After almost 3 years of continuous loadings, the beds were left to rest for a period of 6 months without loadings (mostly in warmer months, June-October), during which regular samplings and monitoring continued. The Cr content in the normal SAS was 276 mg/kg ds, but after the Cr addition, it increased to 5002 mg Cr/kg ds [48]. The dewatering efficiency of the three pilot STRBs exceeded 96% in terms of sludge volume reduction, indicating that the dewatering process was not affected by the addition of Cr [31]. However, the TS and VS contents were up to 10% and 2% lower, respectively, compared to those in other beds receiving the same SLR and SAS without added Cr. The Beds B and C receiving the two lower SLRs showed lower produced reed biomass values and lower evapotranspiration values, but the Bed A with the higher SLR did not show such changes [56]. This implies that the larger volume of sludge and, thus, water available for the plants could compensate, in a way, the possible effect of Cr addition. It should be noted that the plants did not show any visible toxicity signs in either of the three beds (e.g., yellow leaves), although it was obvious that the plant density and reed height in Bed C (low SLR) were smaller to those in the other two beds. The Cr accumulation rate was high in the residual sludge, and after 2.5 years of operation, it reached 3463, 3625, and 3322 mg/kg ds in Beds A, B, and C, respectively [50]. During the final 6-month resting phase, the Cr content was further increased, reaching 8604, 8042, and 7686 mg/kg ds, respectively, due to the further mineralization and reduction of the residual sludge volume. Higher Cr content was found at the bottom part of the residual sludge layer in all three beds than at the top [48]. It was also found that the addition of Cr affected the time needed for the residual sludge to reach good stability values in Bed A receiving the high SLR, resulting in a slower degradation rate and an  
Figure 7: Picture of the pilot STRB in Xanthi (northern Greece) treating activated sludge with a high Cr content. inhibition of the microbial respiration activity [57]. However, in the two beds receiving lower SLRs, sludge stabilization was not significantly affected by the high influent Cr content.