## 4 Leak Detection Technologies
### Hardware-Based Methods
#### 4.1.3. Inline Technologies  
Inline technologies for detecting water leaks in pressurized pipelines have witnessed significant advancements, offering promising alternatives to traditional methods for more efficient and accurate leak detection. Tethered systems, exemplified by the Sahara System, and free-swimming systems, typified by the SmartBall, constitute two examples of inline technologies to detect water leakage in pressurized pipelines. The Sahara System incorporates a hydrophone acoustic sensor for inline leak detection [18]. Operating within a pipeline pressurized between 0.3 and 13.8 bars, the hydrophone is propelled through the pipeline by water flow, detecting leakage noise and marking the leak's location [49]. While capable of detecting leaks as small as 1 L/h and unaffected by pipe material or soil type, the technology is relatively expensive and may face challenges with tether cable path blockages by various factors, such as drag, friction at bends, and the presence of inline valves [50]. In contrast, free-swimming systems, exemplified by the SmartBall, offer an innovative approach. The SmartBall, equipped with an acoustic sensor, accelerometer, magnetometer, GPS-synchronized ultrasonic transmitter, and temperature sensor, silently traverses the pipeline using water flow [19]. Its spherical design eliminates noise interference, allowing the acoustic sensor to operate effectively [51]. The SmartBall can survey extended pipeline lengths, up to 43 km, with specific flow rates and battery life [30]. It detects leaks as small as 0.15 gallons per hour, notwithstanding the pipeline material, and can identify pockets of trapped gas in pressurized pipes. However, the potential for false alarms due to acoustic characteristics resembling real leaks remains a concern. This technology has been tested in several case studies, including a successful survey by the Birmingham Water Works Board (BWWB), which used the SmartBall for inspecting 12 km of 1000 mm reinforced concrete pipe, detecting 26 leaks of varying sizes with close location accuracy and preventing long-term water loss [52]. The application of fiber optic sensor systems represents another inline approach to pipeline leak detection. By installing fiber optic cables along the pipeline's length, changes in the thermal characteristics of the surrounding soil can be monitored to detect and locate leaks [53]. These systems measure temperature and strain along the cable, enabling real-time monitoring to prevent failures, timely detect issues, and facilitate repairs [54]. Additionally, existing fiber optic telecommunication lines may be utilized for temperature monitoring and leak detection [55]. Nonetheless, these advanced methods warrant further research and development to address potential limitations and optimize their effectiveness for various pipeline conditions and materials. Future research on hardware-based methods should focus on refining non-acoustic approaches, acoustic tools, and inline technologies. The continuous refinement of gas injection techniques, including exploring alternative tracers, holds promise for improving leak detectability in varied environments. Research into machine learning algorithms can automate detection processes, particularly in thermography. Ground-penetrating radar technology advancements can enhance resolution and depth capabilities for effective subsurface leak detection. Optimizing negative-pressure-wave systems and exploring smart valves or controls can improve their efficacy. Regarding acoustic methods, signal-processing and machine learning advancements can automate analysis, and compact, cost-effective leak noise loggers are essential for widespread deployment. Inline technologies, such as Sahara, SmartBall, and fiber optic systems, require inquiry into heightened sensitivities, expanded applicability, and innovative methodologies for increased detection precision. These research efforts aim to enhance the water leak detection accuracy, efficiency, and adaptability across diverse water distribution infrastructures.