DEEP TUNNEL DROP STRUCTURE AIR VENTILATION STUDY Northeast Interceptor Sewer and East Central Interceptor Sewer

A sewer air ventilation study and the impact of drop structures on the collection system was conducted to quantify the air ventilation dynamics resulting from a diversion of 30-cfs ADWF of wastewater flow from the North Outfall Sewer (NOS) to the Humboldt Drop Structure on the Northeast Interceptor Sewer (NEIS). The diversion resulted in changes to the down stream flows on the NOS and East Central Interceptor Sewer (ECIS) resulting in changes to the air dynamics occurring in the drop structures and the sewers interconnected to them. In the upper reach on NEIS at the Humboldt drop structure, there were no ventilation impacts. The 30-cfs flow entering the Humboldt drop structure created enough air drag to create negative pressure conditions at the drop structure and interconnecting sewer. The air return line at the Humboldt drop structure was plugged temporarily to measure the impact of the air return line. Plugging the air return line improved the air dynamics in the drop structure. On the ECIS tunnel, changes in ventilation conditions were observed. Mission and Jesse drop structure was the most impacted due to a reduction of flow to 5 cfs to entering the drop structure. The reduction in incoming flows and the air contribution from the ECIS tunnel, resulted in significant air pressurization to the interconnecting NOS. The 23rd and San Pedro drop structure was only slightly impacted from the increased flow on ECIS. The 47-cfs from the NOS entering the 23rd and San Pedro drop structure generated sufficient air drag to counteract the effects of the air ventilation generated from the ECIS tunnel and the drop structure air return line resulting in minimal impacts to the interconnecting sewers.