The present invention relates to a novel economical on-site electrochemical based membrane cell based process with the capability of producing high strength sodium hypochlorite and/or elemental chlorine gas in any ratio as required by the needs of a water or wastewater treatment plant. The system is compact and modular, using membrane cell based electrolyzers and utilizing novel process modifications and sensors to allow for the unattended control and safe operation of the process. The process allows the operator to produce elemental chlorine gas and sodium hypochlorite in any product ratio, such that 5% to 100% of the total chlorine produced by the process can be converted to high strength bleach. The process has the flexibility to produce stable high quality, low to high strength sodium hypochlorite solutions in concentrations ranging from about 2 to 15% trade as NaOCl.