To determine the correct statements regarding the disinfection treatment of water, let's analyze each statement:

1. A. Temperature has an effect in disinfection treatment of water.
   • Explanation: Temperature can influence the rate of chemical reactions involved in disinfection. Higher temperatures generally increase the reaction rates, potentially enhancing the effectiveness of disinfectants like chlorine.
2. B. pH controls the amount of HOCl and OCl⁻ in water.
   • Explanation: The pH of the water affects the distribution between hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). At lower pH levels, HOCl predominates, while at higher pH, OCl⁻ becomes more prevalent. HOCl is the more effective form of chlorine for disinfection purposes.
3. C. Cl₂ gas used for disinfection forms HCl as a product in water.
   • Explanation: When chlorine gas (Cl₂) is added to water, it forms hypochlorous acid (HOCl) and hydrochloric acid (HCl) as follows: \(Cl_2 + H_2O \rightarrow HOCl + HCl\)
4. D. Disinfection action is because of HOCl formed and not due to Cl₂ gas.
   • Explanation: The disinfection effect of chlorine is primarily due to HOCl because it is a more potent disinfectant compared to Cl₂ gas and hypochlorite ion (OCl⁻).
5. E. Combined chlorine exists in water due to combination of HOCl with NH₃ and organic compounds.
   • Explanation: Combined chlorine, also known as chloramines, forms when HOCl reacts with ammonia (NH₃) and organic amines. These compounds can be less effective disinfectants compared to free chlorine.

After considering each statement, all five statements (A, B, C, D, and E) are correct. Therefore, the correct answer is A, B, C, D and E.

This reflects a comprehensive understanding of how various factors such as temperature, pH, and chemical reactions affect chlorine's disinfection efficacy in water treatment.