Last updated on 2025-12-09
By A. W. Wyman, Superintendent, Pasadena Sewage Treatment Plant
The subject of this paper takes us back to many unpleasant experiences, and except for the many benefits others as well as we ourselves derive from them, we wouldn’t care for any post mortems. Trials and tribulations are caused by failures, some impossible to forecast and others by poor judgment or lack of experience.
At the time Pasadena’s plant was started, in January, 1924, the activated sludge process in this country was not very far advanced, so naturally many errors were made, some pretty expensive and others merely operators’ problems.
Initial Operating Difficulties
The first serious one, and one which is so often made even by capable engineers, was lack of capacity. This started a prejudice against the plant which has not been entirely overcome as regards our near neighbors. Many figure that if the average daily flow is provided for, enough energy can be stored during low flow to take care of the peaks. This in my experience is entirely wrong, especially when our peak flow (nearly double our average flow) lasts longer than the four hour detention period which we have found necessary for good results. This lack of capacity included aeration units, clarifiers, sludge return pumps and sludge filters. It had been decided to discharge the effluent into a wash just below the plant, but at the last minute an order from the State made this impossible. The reason for this was that, during the two weeks’ period after starting a plant, the effluent is very little better than raw sewage, and the stability of effluent at all times must come up to state and county requirements. Besides, the wash, for the most part, ran through private property.
Here was the operators’ problem: how to return to the farm property all the effluent, with a pump capacity of about half that amount. The only alternative was to control the raw sewage flow into the plant and bypass the surplus into lagoons or basins. Many thousands of dollars were spent in building basins for this purpose, and it was in the lagooning that the plant got its bad name. New aeration and clarifier units were rushed to completion, and finally an effluent line from the plant to the Rio Honda was built, which removed the necessity of lagooning the raw sewage.
Sludge Disposal
In the meantime the sludge problem was causing more concern than had the raw sewage. High pressure salesmanship, and a wonderful article, appearing in a well-known engineering magazine, contributed by the salesman himself, had sold the city a filtering machine used in the paper industry. This machine was housed in a very fine building arranged for its accommodation only, and which later required much serious study to rearrange for filters that are filters.
Going back to the paper machine, a company engineer was supplied with the equipment to prove their guarantee, that of filtering all excess sludge produced by the plant using SO. gas as a coagulant. After one or two months’ experimenting, this machine was declared worthless, and only by starting suit did Pasadena receive the money paid for it. While all this was going on, sludge was being pumped to lagoons, used for irrigating the orange orchards, and in fact everything which could be thought of to dispose of it. The trials and tribulations during this period are still fresh in my mind. A real nuisance and probably a menace to public health existed. Flies and mosquitos were breeding by the thousands, and irate neighbors and State Board of Health and County officials were insisting upon quick action, our neighbors even going so far as to threaten to blow up the plant. City officials were back of every move to improve the situation, in fact have always been more than liberal in meeting the requirements of the plant. Trenching was finally adopted, and hundreds of ditches were necessary to hold the half liquid sludge which had been accumulating between the failure of the paper machine and the installation of two large vacuum filters. ‘These filters needed many months to bring them to the efficient stage which they now hold. One instance about these first filters I well remember. Cotton cloths costing about $50 each were lasting only 30 or 40 days. These cotton cloths filtered 4,500,000 gallons in thirty days of continuous service. We are now using woolen cloths at $75 each, which last six months and filter 30,000,000 gallons each.
Air Distribution
Another serious and costly error was the manner in which filtrous plates were set. Asphaltum was used for cementing the plates, and many leaks were discovered when the plates were covered with a few inches of water to test them out. Once the plates were wet it was next to impossible to make asphaltum stick and stop the leaks. These leaks increased in number until nearly all the air was coming up around the plates, and uniform air distribution, which is very important, was impossible. This left nothing to do but remove the plates, one tank at a time, and chip off the concrete edges of the holders where the tar still prevented a bond for the new setting in concrete. Each plate was washed in carbon bisulphide, coal oil, and water before it could be used again, and instead of having new plates of uniform porosity, they were now only about 75 per cent efficient. This was some job in itself, there being about 3000 plates. These plates are still in service, but will be replaced with new plates from time to time. It is hoped this may show quite a reduction of the amount of air now used.
The blowers also caused their share of trouble. Check valves of the swing type were responsible for a general scramble for the power house to close a positive gate valve, and save some blower from tearing itself to pieces in reverse gear on account of air rushing back through a stuck check. These have been replaced by cushion plunger type checks, needing almost no attention. A gravity oiling system relieves the operator from the tiresome and almost continuous necessity of hand oiling.
Screens and Settling Tanks
The screen house gives very little trouble since the bucket conveyors have been changed from a two-chain drive to a single chain. The fine screens are very efficient, but I am sorry I cannot say the same about the clarifiers, that is, the type which was furnished the Pasadena plant. These were of the extension arm type which operated from weights and wire cables. The wire cables were replaced many times, until cable chain and chain sheaves were finally furnished. These worked. The extension arms have been discarded and all clarifier tanks (now five in number) have been changed from square to round bottoms, making it possible to try out spiral equipment, which has been very satisfactory. Nearly all these troubles are ancient history, licked and almost forgotten, but there are still one or two quite recent ones.
Drying of Sludge
Burying sludge was using up all surplus waste land, and, besides being a very expensive disposal, began to cut into valuable farm land. Sun drying was out of the question in the present location, so a sludge dryer was installed, and tested out in February, 1927. The dryer was a direct indirect type, 70 in. diam. and 42 ft. long, with a Dutch oven using fuel oil under 70 lb. pressure. An exhaust fan of 8000 c. f. m. gave the forced draught necessary and carried the gases and steam away through a coke filter. ‘The guarantee of this dryer was that it would dry 500 lb. per hour of feed sludge, containing 83 per cent moisture, and that no odors would
result. But 300 lb. was the very best the dryer would turn out, and as to odors, there were plenty. The coke filter, heavy water sprays, and finally chlorine as high as 150 lb. per day, gave no relief. In addition to these, we tried blowing the exhaust gases up the incoming sewer line, but they escaped from manholes and the odor was still as bad. Then it was turned down the effluent line, with no outlet until it reached the Rio Honda, four miles away. This gave promise of some relief, but was soon reported to county and health authorities as causing death and destruction to people living half a mile away from the discharge. An ozone generator of 12 grams per minute capacity had been given credit for a slight improvement, so another ozone machine of double the capacity was purchased on a guarantee that all odors would be eliminated or no money. Both machines are still in storage, and the second has not been paid for. A compromise was made on the dryer, on the basis that it should deliver three tons per 24 hours instead of six, as guaranteed. This left a real problem for the operator. Dryer capacity was less than half that required by the plant. Many changes were made in the dryer. It was lengthened to 60 ft., a new circular fire box was built, also a screen and return conveyor for oversize material. ‘This stepped up the output to 4 1/2 tons per day, still not enough. Experiments with a filter aid almost immediately brought the results we desired. Filter capacity was increased nearly 50 per cent, and the moisture of the sludge cake was reduced more than 5 per cent. The effect on the dryer was very pronounced, jumping the capacity to 8 or 10 tons daily. This was very gratifying as far as the dryer was concerned, but the odors were still causing many protests; and finally an order came to close down the dryer until some means could be worked out to eliminate the odors.
Summing up everything that had been tried without success—condensers, water-washers, sprays, deodorizers, chlorine and ozone—we next tried superheating the steam and passing it through a separate fire-box. This fire-box is about 6 ft. wide and 20 ft. long, having an outside shell through which the gases and steam pass before going into the combustion chamber itself. There are over-and-under baffles which insure contact for all gases, and a 20-ft. stack discharges them into the outside atmosphere. This arrangement seems to be exactly what was needed for the elimination of these odors, and I believe is the solution of the most persistent of all our problems.
This paper possibly has left an impression that everything must have gone wrong at the Pasadena plant, but such is not the case, for many installations have worked to perfection from the start. Looking back over the last five years, the satisfaction of permanently solving these many problems makes the trials and tribulations of the sewage plant operator not so serious, after all.
(the article goes on to talk about operating expenses which were obtained after the presentation and I have excluded them because I am so lazy. Click the link to read them at the source)
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