Purification Problems of a Turbid Water Supply
Results Obtained from Water Purification Plant at Topeka, Kan., After Two Years of Service—Difficulties Overcome by New System
THE problem which the city of Topeka was compelled to meet and solve in the treatment of its water was a particularly difficult one, owing to the condition of the raw water supply taken from the Kansas River. The following article gives a very clear description of these difficulties and how they were met:
The water purification plant at Topeka, Kansas, has now been in operation for approximately two years and an article descriptive of the results obtained from the treatment of the hard, muddy and polluted Kansas River water may be of interest at this time. The plant consists of aerators, grit chambers, mixing chamber, settling basins, pre-filters, coagulating basin, final filters and sterilizing apparatus. Equipment is provided for feeding alum, lime, soda ash and chlorine, so that the plant can be operated either for the clarification of the turbid river water alone or for clarification combined with softening.
The nominal capacity of the plant as regards preliminary treatment, i.e., aeration, settling, mixing, and coagulation is eight million gallons per day, and filter equipment is provided for a normal rate of six million gallons per day.
The character of the water of the Kansas River is extremely variable and this stream as regards water supply may be classed as hard, turbid and polluted. In the following table the average monthly turbidities, alkalinity, total hardness, bacterial count and B Coli index are shown and these figures represent conditions familiar to most of the purification plant operators of the Middle -West.
Day to day and even hour to hour variations in turbidity and other characteristics are frequent and often wide, and constant vigilance in adjusting chemical feeds to existing conditions is a prime requisite for successful operation.
During 1923 a total of 1,475,250,000 gallons of water or an average of approximately 4,040,000 gallons per day were treated, at this plant, of which approximately 97.8 per cent. or 3,951,000 gallons were pumped to the city.
The following table shows the monthly average rates together with the maximum each month:
In 1920 the City of Topeka used an average of 3,600,000 gallons per day with a maximum of 4,700,000 gallons and the increased present consumption approximately 12 per cent. represents in part the normal growth of the city but is also due to the fact that the present water supply is entirely adequate and more popular than the old ground water supply which contained considerable iron and was quite hard.
Low Service Pumpage Operation
Water is pumped from the river or from the old system of wells if desired, into the aerator by means of motor driven centrifugal pumps, through a discharge line which is fitted with a float controlled valve for the purpose of maintaining the water level in the basins at the proper elevation.
The aerator is of the multiple pan type with a total weir length of 900 feet and a drop of 15 inches. With river water the principal service of the aerator is the elimination of odors from decayed organic matter which are at times quite evident in the raw water and the aerator is also of service in removal of carbon dioxide front the raw water preparatory to softening, but due to the usual high turbidity of the water and the resulting difficulty in making dissolved oxygen or carbon dioxide tests no attempt has been made to determine its actual efficiency.
The grit chamber which was designed for a detention period of one hour (at an eight million gallon rate) has during 1123 actually had an average period of from 111 to 126 minutes and as a result more sedimentation has taken place here than was intended. A large amount of relatively large sized grit and sand is pumped up with the raw water and this makes the sediment from this basin hard to handle.
The floor of this basin is filled with underdrains which terminate in one and a quarter inch openings in the floor spaced five feet, on centers, both ways, and which lead to header pipes fitted with quick opening valves.
The system has not proven an unqualified success due to the relatively large amount of grit and it has been found necessary to empty this basin several times and flush out the heavy sediment with fire streams, but it has saved a considerable amount of time and water since with this system, the period between flushings is materially longer than would be the case if flushing alone were relied upon.
The mixing chamber has a capacity of approximately 180,000 gallons and is intended to provide a period of 30 minutes at the nominal rate of eight million gallons per day but under actual conditions during the past year, this period has been from 60 to 71 minutes. Thorough mixing is obtained by the operation of motor driven agitators or paddles and by the flow of the water itself through the baffled basin.
This system of mixing while entirely adequate, has two defects. First the normal wear and tear on gears, bearings and other mechanical parts, and second, the tendency of dissolved lime to accumulate in the corners formed where the baffles join the walls of the basins, which accumulation tends to clog the underdrains. Care and attention is required for the mechanical parts and the second defect has been overcome by placing water jets in the corners where the accumulation occurs to keep the sediment in motion.
Chemical Feeding Equipment
Alum is fed in the form of solution through orifice boxes so located that it may be applied at the entrance of the mixing chamber, the entrance to the settling basins and the entrance to the coagulating basin.
Lump alum is used and the solutions are made up in two wood stave solution tanks located on the second floor of the plant.
Hydrated lime is fed through two dry feed machines fitted with steel hoppers which permit of feeding this dusty chemical from a separate room on the second floor. The application of lime is made at the entrance to the mixing basin only.
Soda ash when used is fed through dry feed machine direct into the mixing basin.
Motor driven agitators are provided in the hoppers of the lime and soda ash machines to prevent “caking” of the chemical.
From the mixing chamber the water flows by gravity into the settling basins, two in number with a combined capacity of 1,950,000 gallons or six hours flow for the nominal capacity of eight millions and from 10.6 to 12.5 hours flow for the draft during 1923. The basins are operated in parallel and are filled with skimming weirs at each end and “A” frame baffles in the center. The water depth is 16 feet and the average velocity in the basins during 1923 from .15 to .18 feet per minute.
The settling basins are filled with underdrains similar to those in the grit basin and due to the more finely divided condition of the sediment their operation is much better. There is a tendency for them to clog, however, at the inlet end of the basin and at the extreme ends of the laterals, and it has been necessary to flush the basins out with fire streams at intervals of approximately six months.
The roughing filters are three in number and are similar in construction to the final filters with the exception that the filtering material is an 18 inch layer of one-quarter inch screened Joplin chats, laid on a graded ground bed.
These filters are operated at a rate of 4 gallons per minute per acre or approximately double the rate of the final filters and while little or no clarification of the water is effected, the real purpose of the filters, i.e., the removal of excess carbonate hardness, is fulfilled as shown by the fact that during 1923, the carbonate alkalinity of the water applied to the filters as shown by the monthly average results did not exceed 13 parts per million.
The coagulating basin, which receives the water from the roughing filters and delivers it to the final basin is similar in construction and size to the grit basin except that it is baffled.
(Contniued on page 36)
Purification of Turbid Water Supply
(Continued from page 14)
The time of coagulation is from 110 to 130 minutes under present conditions which time will be reduced to 60 minutes when the capacity of the plant is reached. The present velocity within the basin is approximately 3.3 feet per minute.
The final filters are six in number, each with a nominal capacity of one million gallons per day. They are hydraulically operated throughout and of standard construction, except for the underdrains in which perforated pipes have been substituted for the standard strainer system.
Excellent results have been obtained with these filters since they were put into operation and in 1923 the wash water used amounted to 2.2 of the total. The average turbidity of the settled water for the same period was 73 parts per million.
In the fall of 1923 the city installed two electrolytic cells of the Williams type and these were operated during November and December and, intermittently since that time.
(A full description of this system was published on page 1263 of the December 10, 1924, issue of FIRE AND WATER ENGINEERING.-EDITOR.)
The water purification plant is under the supervision of a chemist, and the operating force consists of three operators, each of whom has an eight hour shift. Extra help for maintaining the grounds, buildings and equipment, cleaning basins or other work not connected directly with the operation of the plant is furnished as needed by the water department.
The laboratory tests for the control of the plant are as follows:
- Turbidity—Run on raw and settled water daily or oftener if the condition of the water seems to be changing.
- Alkalinity—Run on raw, settled and filtered water daily.
- Carbonate Alkalinity—Run daily, or oftener on settled and filtered water.
- Total Hardness—Run daily on raw water and filtered water, by soda reagent method.
- Free Chlorine—Run on sterilized water daily or oftener. (OrthoTollidin method).
- Bacterial Counts—Run daily on raw, settled, filtered and (Nutrient agar) sterilized water. Plates (encubated) 24 hours at 37 degrees C.
- B. Coli—Run daily on raw, settled, filtered and sterilized water. (Presumptive tests in Lactose broth read at 24 and 48 hours’ encubation at 37 degrees C).
- Mineral Analysis—Run on raw and filtered water each month on a composite sample made up of portions of each day’s run.
The proper amount of the various chemicals to be fed is determined each day by the chemist and his instructions are placed on a black board in the operating room.
Daily records are kept on the operation of the filters, the chemical feeding rates and analytical results, and the quantity of water treated and these records are summarized each month for the permanent plant record.
When the plant was put into operation it was decided to produce a treated water with not to exceed 135 parts per million of hardness, but on trial it was found that the sudden change from the hard well water was not popular and resulted in so many protests from the water users that the softening was partially eliminated with the intention of increasing it gradually so that the effect would not be so noticeable.
The old supply, as is usual with hard iron waters, had a noticeable taste and the change from this to softened water with little or no taste, caused many complaints of “flat taste,” “druggy taste,” etc. During 1923. however, the total hardness of the treated water was gradually reduced from 167 to 127 parts per million and no complaints have been made.
(Excerpts from paper read before the annual convention of the Iowa Section, American Water Works Association.)