# What Advanced Treatments Can Be Used to Minimize the Production of Sewage Sludge in WWTPs?
## 3 Advanced Treatments and Technologies
### Biological Treatments  
#### 3.2.1 Membrane Biological Reactor (MBR)  
In MBR systems, the separation of the sludge from the effluent takes place in highly efficient membrane modules instead of the traditional secondary sedimentation [59; 60]. The expected production of sludge in an MBR system is usually lower than in CAS systems, due to the high concentration of biomass in the tank (7-20 g\({}_{\text{TSS}}\) L\({}^{-1}\)), the low Food/Microorganism (F/M) ratio and, consequently, a high sludge retention time (SRT). Therefore, under these conditions, cell growth is not promoted [25]. \(\bullet\) Mesophilic Membrane bioreactor (MMBR)  
In recent years, MMBR technology has been proposed as a suitable alternative to CAS because of its higher effluent quality, smaller volumes and a lower amount of sludge production [61; 62; 63]. The mesophilic temperature conditions vary in a range between 20 and 45 \({}^{\circ}\)C [64]. Being an MBR, this process can be considered a hybrid of a conventional biological treatment system and physical liquid-solid separation using membrane filtration [65]. In MMBR, differently from CAS, the reduction of sludge production appears to be due to the development of predators in the tank, but the opinions on this are discordant [25]. The MMBR technology provides the following advantages over CAS: higher quality effluent for water reuse since bacteria and suspended solids will be retained by membrane, higher volumetric loading rates, shorter hydraulic retention times, longer solid retention times [65; 66; 67]. Another advantage of MMBR over CAS systems is the small space required due to the absence of a secondary clarifier and the higher volumetric conversion rates [65; 68]. The higher volumetric conversion rates are the result of higher biomass concentrations [68]. However, membrane fouling, attributable to high concentrations of TSS, colloids and sludge flocs, remains a significant drawback of MMBR, as it significantly reduces membrane performance and useful life [25; 66; 69]. It leads to an increase in operation costs and a limitation in the transfer of oxygen [25]. Moreover, sludge derived from CAS and MMBR systems results in different dewatering efficiency according to [PERSON] et al. [70]. This aspect has been studied by [PERSON] et al. [61]. The achieved results confirmed the complexity of the inter-relationships between many factors affecting the sludge dewaterability. [PERSON] and [PERSON] [71] confirmed that the literature reports differ and there are contrasting data on the best or worst sludge dewaterability from MMBR compared to that from CAS. However, [PERSON] et al. [70] affirmed that MMBR configuration produced a sludge with a lower mean particle size respect to the CAS and, therefore, worse dewaterability properties. On the contrary, [PERSON] et al. [72] tested the effect of anaerobic digestion on dewatering properties in MMBR and CAS. They found that worsening effect due to the post anaerobic digestion is decidedly lower for MMBR sludge with respect to CAS. Several authors also investigated the possibility of adopting suitable sludge retention times in the tank in order to achieve zero net growth [73; 74]. Moreover, in the last decade, MMBR systems have been coupled with other disintegration treatments (e.g., ozonation, ultrasonication) in order to increase the biomass decay rate, maintaining a relatively low concentration of TSS in the MMBR system [25; 75; 76]. [PERSON] et al. [25] reported that these applications, which can only be used in the water line, make it possible to achieve a reduction of 99% in terms of TSS. \(\bullet\) Thermophilic Membrane Bioreactor (TMBR)  
The TMBR represents an evolution of MMBR. In this case, a membrane system (generally ultra or nano-filtration) is coupled to the thermophilic biological process. This aspect allows us to overcome the problem due to the poor settleability characteristics of the thermophilus biomass. Thermophilic microorganisms show optimal growth at temperatures of 50 \({}^{\circ}\)C or higher [77].