nnnnnn 



1 • I Contact Chamber 

 \\ \ 

 \\\\ 



1 l' ! ill ,-. , r-.,, 



, j ■ • | rinal effluent 



Intro, fig. 52. Model sewage treatment plant showing the 

 various processes including settling, filtration, and sludge 

 digestion. The arms on the filter rotate slowly, spraying fluid 

 which trickles over the zobgloeal surfaces and through the 

 interstices of the rocks (Eliassen, 1952). 



industrial wastes is by oxidation in shallow ponds. 

 This is the procedure used in many suburban and 

 rural parts of California. Sewage is settled and 

 strained to eliminate solid matter and then the fluid 

 is run through a series of shallow ponds several 

 acres in extent. Organic matter is acted on by bac- 

 teria, first under virtually anaerobic conditions and 

 later in the presence of oxygen supplied in large 

 part through photosynthesis by symbiotic algae. 

 Insects play an important part in this system. Only 

 surface breathing Tubifera larvae and other Diptera 

 with respiratory tubes can live in the milky, anaerobic 

 water of the first ponds. Later, however, a rich insect 

 fauna develops with many beetles, true bugs, dragonfly 

 and mayfly nymphs, and mosquito larvae. 



By far the most numerous forms, however, are 

 Chironomid larvae (Glyptotendipes, etc.). Counts of 

 1,500 or more per square foot of bottom surface have 

 been made, and the total dry weight of the annual 

 crop of larvae in a 4% acre pond at Concord has been 

 calculated at 575 pounds (Kellen, 1955). The algal 

 blooms utilize end products of bacterial metabolism 

 and form a high percentage of the final effluent. 

 However, the larvae extract algae by filter feeding; 

 also, they burrow at die mud-water interface, extend- 

 ing oxygen deeper and promoting the exchange of 



37 

 Usinger: Introduction 



oxygen and nutrients between mud and water. In the 

 biodynamics of oxygen pond treatment insects repre* 

 sent one of the end points of energ) transfer — unle 



fish are introduced to oat the insects. Also m 

 are the only agents that actually remove part of the 

 energy and organic load from the system us the) 

 emerge and fly away. The rest of the unsettled mate- 

 rial is transformed into (load algal cells with a B.O.D. 

 that may not be very different from the original sewage 

 and hence creates a considerable load when dis- 

 charged into a stream. 



Detection of pollution. — Various tosts have boon 

 devised to recognize the presence of pollution and to 

 detect the effects of past exposure to wastes. The 

 simplest methods and therefore the ones most gener- 

 ally used are physical and chemical. It is relatively 

 easy to take water samples from a stream, for example, 

 and test for dissolved oxygen (DO), biochemical 

 oxygen demand (B.O.D.), pH, turbidity, and the pres- 

 ence of toxic chemicals. However, such tests are 

 not very revealing because pollutants are seldom 

 discharged continuously and therefore their presence 

 may be missed by sampling at the wrong time. 



What is needed is a method of determining the 

 effects of pollution on the resident biota, and this 

 is precisely where insects fit into the picture. 

 "Specifically (as stated by Gaufin and Tarzwell, 

 1953), the degree and extent of pollution in a stream 

 can be determined accurately by reference to the 

 macroinvertebrate fauna, particularly that found in 

 the riffles. A biological analysis of the pollutional 

 status of a stream can be obtained in the field through 

 recognition of the orders, families, or genera in the 

 invertebrate associations encountered. This type of 

 biological inventory is superior to chemical data, 

 [because] the complex of such organisms which 

 develops in a given area is . . . indicative of present, 

 as well as past, environmental conditions . . . Bottom 

 organisms are more fixed in their habitat than are fish 

 or plankton and cannot move to more favorable sur- 

 roundings when pollutional conditions are most 

 critical." 



Because of the importance of the bottom fauna 

 and more particularly in streams, the insect fauna, 

 attempts have been made to set up criteria of abun- 

 dance or to fix upon the presence or absence of 

 certain indicator organisms as evidences of pollution. 

 Unfortunately, these efforts have failed because, as 

 pointed out by Patrick (1953), it is the total spectrum 

 of all groups of organisms that provides the best 

 criterion for judging the "health" of a river. In 

 general a normal stream will be rich in species with 

 no single group predominating, whereas a polluted 

 stream is poor in number or variety of species but 

 often rich in individuals. Therefore it has been sug- 

 gested (Needham and Usinger, 1954) that biological 

 sampling for detection of pollution be done by means 

 of bottom samples (Surber or drag-type samplers). 

 The organisms in the samples should be sorted out 

 according to orders or other major taxonomic groups. 

 In this way the spectrum of organisms can be deter- 

 mined with only two or three square-foot samples 

 giving reliable evidence as to the presence or absence 

 of main groups. 



