A wastewater treatment system for temporarily storing and treating excess wastewater discharged from a sewer system generated during a sufficient rain event, the wastewater treatment system comprising: a first storage chamber having an inlet adapted to receive excess wastewater directly from the sewer system, the first storage chamber being oriented generally vertically having an upper end and a lower end, the upper end having an overflow outlet and the lower end having a return outlet; a second storage chamber associated with a third storage chamber being oriented generally vertically, each having an upper end and an inter-connected lower end, the second storage chamber having an inlet in the upper end adapted to receive wastewater from the overflow outlet of the first storage chamber, the upper end of the third chamber having an effluent discharge passage overflow outlet, and the lower end of at least one of the second and third chambers having a return outlet; wherein, the collective volume of the first storage chamber second storage chamber and third chamber is over 37,854,000 litres (10,000,000 gallons); a treating agent dispenser for introducing a treating agent into the wastewater in at least one of the second storage chamber or third storage chamber, said treating agent for at least partially disinfecting the wastewater prior to entering the effluent discharge passage overflow outlet; at least one return pump (26, 28) having an inlet connected to the return outlets and an outlet connected to the sewer system for returning any wastewater in the first storage chamber, the second storage chamber and/or the third storage chamber to the sewer system via the return outlets upon completion of the sufficient rain event. a first storage chamber having an inlet adapted to receive excess wastewater directly from the sewer system, the first storage chamber being oriented generally vertically having an upper end and a lower end, the upper end having an overflow outlet and the lower end having a return outlet; a second storage chamber associated with a third storage chamber being oriented generally vertically, each having an upper end and an inter-connected lower end, the second storage chamber having an inlet in the upper end adapted to receive wastewater from the overflow outlet of the first storage chamber, the upper end of the third chamber having an effluent discharge passage overflow outlet, and the lower end of at least one of the second and third chambers having a return outlet; wherein, the collective volume of the first storage chamber second storage chamber and third chamber is over 37,854,000 litres (10,000,000 gallons); a treating agent dispenser for introducing a treating agent into the wastewater in at least one of the second storage chamber or third storage chamber, said treating agent for at least partially disinfecting the wastewater prior to entering the effluent discharge passage overflow outlet; at least one return pump (26, 28) having an inlet connected to the return outlets and an outlet connected to the sewer system for returning any wastewater in the first storage chamber, the second storage chamber and/or the third storage chamber to the sewer system via the return outlets upon completion of the sufficient rain event.