Low Power Wide Area Networks (LPWAN) which provide low-power consumption, long-range communication, low data rate is becoming the powerful communication technologies of the IoT of tomorrow. LoRaWAN, SigFox, and NB-IoT are the three competing LPWAN technologies. On the other hand, Smart Water Grid (SWG) is an emerging paradigm that promises to overcome issues such as pipes leaks encountered by current water infrastructure by deploying smart devices into the water infrastructure for monitoring purposes. This paper firstly explores the physical and communication features of the above LPWAN technologies and provides a comprehensive comparison between them as well as their suitability for the Smart Water Grid (SWG) use case. The important aspect of SWG is to connect devices such as smart water meters and other tiny devices like sensors installed into the water pipelines for the system monitoring purpose. Often, the number of devices need to connect is high due to the vastness of the water system, and the most scalable communication technology is required. Therefore, we consider Advanced Metering Infrastructure (AMI) also called Smart Water Metering when dealing with the water grid, which is the main application of SWG and we study the scalability of LoRaWAN, NB-IoT, and SigFox in such application. Under NS3, the simulation results show that NB-IoT provides the best scalability compared to LoRaWAN and SigFox and thus is able to support a huge number of devices with a low packet error rate. Additionally, the results show that the increase in the number of gateways leads to the reduction of packet error rate and thus gives the capacity to the network to support a great number of devices.