10 LIMNOLOGY, WATER SUPPLY AND WASTE DISPOSAL 



the "biological gradient" could be demonstrated. Near the influent of the 

 primary aerator in one instance, the growth was very heavy, filling the 

 spaces between the plates in places. This condition was accompanied by 

 an unhealthy growth of the bacterial slime, Zoogloea ramigera, and the 

 presence of numerous anaerobic flagellate protozoans. Myriads of green 

 and red sulphur bacteria as well as the green alga Chlorella vulgaris, and 

 the diatom, Navicula sp. occurred where the light could reach them. Free- 

 living bacteria were also abundant in this part of the aerator, but no 

 ciliate or amoeboid protozoans were to be found. Near the outlet of this 

 aerator, however, there were large numbers of ciliate and flagellate proto- 

 zoans which were subsisting largely on bacteria in the sewage. In the 

 secondary aerator, the microbiota showed continued improvement. The 

 appearance of the slime on the plates indicated a healthy condition, and 

 dense pink colonies of rotifers occurred on the upper portions of some plates. 

 Moreover, large numbers of Opercularia sp. occurring in the secondary 

 aerator, indicated that an advanced stage of purification of the sewage 

 had been reached. 



Among the plants of this type there were several instances where 

 overloading caused a failure of the process, as was evidenced by oxygen 

 depletion in the aerators and a septic condition throughout. Anaerobic 

 bacteria and protozoans thrived in a manner similar to what is expected 

 in a digestion chamber for sludge. According to Lackey and Dixon (1943) 

 there was ". . . oxygen starvation in the midst of plenty." Sometimes, where 

 such difiiculty was encountered, the biological gradient was modified by 

 interposing a trickling filter in the system or recirculating some of the flow. 

 In every case, the results were good. We are to conclude from this that 

 the maintenance of the aerobic activity among the microbiota is as impor- 

 tant in a sewage plant as it is in a stream, if a condition of nuisance is 

 to be avoided. 



Trickling Filters Modified for Backwashing 



The microbiota in the trickling filters at South St. Paul is abundant 

 and varied. The filters are not housed; therefore the sunlight on the sur- 

 face rock stimulates the growth of algae and diatoms. Chief among these 

 are Chlorella sp., Oscillatoria tenuis and Navicula spp. Moreover, some 

 organisms were found in a sample taken during backwashing operation 

 that are rarely found in trickling filters. These are Macrobiotus, the water 

 bear, Aelosoma hemprichi, Pristina sp., and Dero sp. The first is a Tardi- 

 grade, the other three are aquatic annelids. Although snails have often 

 been reported from low-rate beds with intermittent dosage of sewage, their 

 occurrence on high-rate units, such as these at South St. Paul, is unusual. 

 Several Physa sp. have been found at a depth of 1-2 feet. In addition to 

 these rather unusual organisms, there are representatives of the higher 

 bacteria, fungi, protozoa (rhizopods, flagellates and ciliates), rotifers and 

 insect larvae. 



Each filter is backwashed every two weeks with 450,000 gallons of 

 plant effluent applied through the underdrains of the filters. The action of 



