|Title||"Design for a Sanitory Sewer System and Disposal Plant Consisting of..."|
|Author||John Caskey Cotton|
"Design for a Sanitory Sewer System and Disposal Plant Consisting of Primary Settling Tanks, Sludge Digestors and Sludge Drying Beds for Chevy Chase, MD, Population 15000"
by John Caskey Cotton
"Submitted to the Faculty of Rensselaer Polytechnic Institute as a Partial Requirement for Degree in Civil Engineering"
Maps and sketches
The purpose of this design to to find the best possible method of sewage treatment for the conditions existing in Chevy Chase, Maryland.
Up until recent years practically all sewage plants designed and constructed have been either of the Imhoff tank construction, which consists of one deep hopper bottom tank, or of the activated Sludge construction. In both of these types the sludge is digested and stored In to bottom of the tank making them deep and at the same ise very often making the cost of construction high. In
tion, these tanks can have no control or very little control of sludge heating or the rate at which It digests.
After careful consideration of these types of plants to the conclusion that a Separate Sludge Digestion type could be the best suited for the existing tins. This type consists of a primary settling tank, solids are separated from the more liquid part. This solid part, called sludge, sinks to the bottom of the tank means of a collecting device is collected in hopper tanks at one end of the settling tank. From there the sludge is taken to the sludge digester where it digests under control of heat and time. The digested sludge is taken to sludge drying beds where it is thoroughly dried and then idried and then properly disposed of by selling it any possible harmful bacteria from entering the river with effluent from the tanks.
No trickling filters will be used as the size and flow the Potomac River Is sufficient to take Dare of the sible small amounts of harmful bacteria which may enter it. Because of the chlorine treatment this will hardly be true, however.
Covered Sludge Drying beds will be used, as It has fond that thi, this cuts the required area In half and sludge can be dried at all time of the year regardless of weather conditions.
In heating the sludge digestor, the gas from the sludge is collected by means of hoods, purified and used as a boiler for heating. This has proven very satisfactory in all plants where It has been used, and it also utilizes the otherwise wasted gas.
This plant has been designed for five years in advance with sufficient additional land to double its capacity.
All of the sewers designed are in accordance with the State Specifications. The topography of the town is such, that approximately three fifths of the total sewage entering the plant has to be pumped to the high point in the town and from there flows by gravity to the plant. All the sewage which has to be pumped from the lowest point to farmers as fertilizer or by using it as a fill.
The effluent or water, from all tanks is allowed to into the river. Thle to not true in all cases as some plants. require tricking filers axed final settling. tanks depending on the conditions of the surrounding territory. In this design no trickling ftl ere or final settling tanks will be use.
This type of plant was chosen as the best design for this problem because 1) there is a layer of rook, 25 feet below the ground surface and if Imhoff tanks were used it would require expensive excavation in rock. The depth of the primary tanks: used at the deepest end is 17 feet. Likewise the depth of the sludge digestor below the ground is 14 feet. 2) It has been found from experience that the best temperature for sludge digestion is 80ºF, and as the average sewage temperature is 59 degrees, it becomes advisable to heat the sludge. In this type of plant the temperature of the sludge can be maintained at this temperature all the time.
Because of the flat topography of the ground at the location of the plant it to necessary to pump the raw sewage from the sewer invert to the level of the primary tank to the sludge from the primary tank to the sludge digester and home to the sludge drying beds.
The raw sewage to chlorinated In the wet well of the house, under Maryland State Specifications, to avoid the highest is collected by gravity flow at a central point in order to avoid more than one pumping station.
The sewers have been designed for thirty years in vanee considering the necessary present sewers plus adds additional lines when sufficient population warrants them. filitration of water into pipes have been allowed for but this territory Is of fairly dry clay and look joint pipes a to be used, 5000 gallons of infilitrratton water per mile pipe per day has been allowed in the design of the pipes is may seem low to some engineers but from studies conducteted by the Washington Sanitary Commission this Is more an a sufficient amount to allow.
All pipes in which ch the water to flowing by gravity ve been designed to flow half full. Pressure pipes are steed to flow full. All pipes up to .24 inch diameter are trif ed.::pipe and above that cast iron. All pressure pipes a cast iron.