What are the main reasons of concrete deterioration in waste water treatment plant?

The problem with concrete structures in WWTP starts with bad design. 
The first problem usually is lack of control of cracks, in my opinion structures should be calculated to crack width smaller then1 mm.
Eurocodes can be used for this calculation (retaining water structures).
Second usually designers forget about environmental class of exposure of the concrete formulas and determine the concrete class only due needed resistance.
For WWTP concrete should resist to  chemical attack from sulfide ions.
This usually results in concrete classes not smaller then C30/37 or C35/45 according to EN206.
How many WWTP have you seen with concrete structures with C25/30 ? Lots I guess.
Then concrete permeability and reinforcement covering should also be controlled, minimum covering should be bigger then the concrete permeability. 
WWTP concrete structures start to deteriorate when the crack with is enough that allows the liquid to penetrate until the reinforcement and start corrosion process and so on..
Chemical attack its also a big problem so make shore that the concrete can actually resist to it! Protective paintings can help but they deteriorate after2/3 years and have expensive maintenance.
  

من اسباب تاكل الطلمبات والمواسير هودرجه الحموضه والقلويه لذلك لابد من المعالجه

Reason can be

1. Ignorance / poor hydrotest during commissioning. 

2. Improper coating if the structure is in contact with harsh chemicals. (protect by use of Venyl Ester / FRP coating)

3. If the plant is constructed in seismic zone and not keeping in mind of the earth activities. 

4. If any modification or maintenance activities involving puncturing of concrete then it may create problem at later stage.

 

high content of alkaline or acids and the presence of bacteria  in the water of waste water plant can lead to loss of concrete compression strength and concrete disintegration , it also can lead to the corrosion of the reinforcement metal re bars usually water that has a PH value of greater than 6 can be used in the concrete mix but , a ph balance is not conclusive to gauge the  water acid content . 

therefore a more conclusive test that gauge the totality of the acidic contamination must be conducted .

complex attack from outside (..........) and inside(.................................) the system

Conditions are appropriate for decomposition Agoaú and be gas h2s and desolved in water to h2so4 The absence of flow suitable for the flow of a small tube Lack of ventilation oxygenating Weakness in the process of packaging and insulation Weakness in the concrete mix Non-completion of the treatment process

السلام عليكم من الأسباب الجذرية للتدهور الخرسانة في محطات تحلية المياه هو تآكل حديد التسليح و ذلك للاجتماع عدة عوامل منها: وجود الرطوبة والأكسجين، تركيز الأملاح الذائبة في المياه ، درجة قلوية اوحموضة الماء PH و انخفاض جودة الخرسانة.

High Chloride levels (this can be measured via online probes of electrical conductivity)

السلام عليكم من الأسباب الجذرية للتدهور الخرسانة في محطات تحلية المياه هي اللسباب التي ذكروها وكذلك انواع من البكتيريا التي لها القدرة العيش في وسط مالح

Because waste water tends to break the microstructure of the concrete biolologically.