222 W MARINE VIEW DR 2016-01-01 MF Import

icles probably does �n in grit chambers ;ne because oF the h and shoft deten- of the nature of the rticularly important :hamber efficiency. it the scouring pro- the proper velocity t. This may be ex- :t there is a critical ,vhich particles of a nsity, once settled, in motion and re- eam flow. An ex- ed =° for the move- �rials by a flowing locity at which par- md speci8c gravity Y� �C, — 1)d]°•� (9) � cm/s (0.03 for grit cham- xfficient, (0.06 for nstant, gra��ity, and :r, cm. follo��•ing formula formula; 1� d�o.a (10) ,fps gra�•ity, and :r, mm. n Equations 9 and r oE the particle to id 10, the critical irtic'.es 0.2 mm in specific gravity oF �,�, is 0.43 m/s (O.i5 fps). In actu�l practice, designs are generally based on controlling the celocih• of flo�v �cithin the range of 03 to 0.�1 m%s (0.75 to 1.2i fps), 1n�1 as close to 0.3 m/s (1.0 fps) as pussible. To appreciate the importance of the effect oE bottom scour on the operation of a grit chamber, it must be recognized that there ��'i11 ah�•a��s be a certain amount oE organic putrescib]e particles that ��'ill setde out ��'ith the grit. In periodt of lo�e Aow this problem ��'ill be substantia]]y «'orse thAn during periods of high 8o�v. \lainte- nance of velocities at approcimately 0.3 m/s (1 fps), ho�ce��er, ��ill permit scour- ing of the grit and the removal from the bed of substantial quaotities of organic putrescible material. In some non-mechanical grit chambers, engineers have pro�•ided removable parti- tions across the storage section oE the channel, or a remocable floor grating be- hceen the storage section and the 9oa'- through section, in an attempt to control scour. Ttds plan ��•ould seem to have merit as a means of reducing grit carn•�over where velocity control at high rates oF flo�v is not satisfactoq•. Such facilities, on the other hand, ��•ould substantially reduce the grit washing effect oE scour during normal Hows. CRIT REJIOV.aL i. Canstruction and condition oE se��•er s}•stem. S. Soil and ground�vater characreristics. 9. Industrial ��•astes. 10. Relatice use oE household garbage grinders. Quantity nnd Compasition oF Grit The quantity and composition oE grit in wastewater streams ��'�11 cary• ��ith the fol- lowing considerations: 1. Type of collection sy�stem (separat or combined). 2. The percentage of the total s}•stem i the combined category, the t}'pes of inlets catch basins, and maintena:�ce procided and the amount of storm ��•ater diverted a overflow points. 3. Types of street surfaces encountere and maintenance provided. 4. Relative area sen•ed (in addition t population ). 5. Climatic conditions (frequenc}• street sanding). 6. Sewer grades. The significance of each oE these factors is ob�•ious. For instance: unimproved streets ��ill contribute more grit than paved streets; icy streets with frequent sanding ��•ill increase grit loadings tremendouslp; properly designed and adequately main- tained catch basins in combined systems �cill teliece the grit load; as inereased amounts oE ��astewater in rnmbined sewers ace diverted to the wastea•ater treatment plant, grit volumes wi11 increase, parricu- larly during heavy rainFal]s; poor se��•er grades ��'ill tend ta collect grit solids, to Ue ��':+shed do�cn to the plant in surges; se��•ers ��'ith deteriorating joints laid ia fine sand and in the ��ater table ��'ill tend to create grit problems; manhale covers located belo��• street grade, particularly in the berm, ��•i11 increase grit loadings; the number oF commercial car ��•ash racla can increase grit ]oadings ;igni6cantly; indus- trial ��•astes, highly concentrated and in large volumes, may change the character of grit completely from ��•hat might be eapected normally; household garbage grinders can change the characteristics of grit significantly. Unfortunately, there are no formulas by �chich the foregoing factors may be evalu- ated properly to give a reliable estimate of anticipa[ed grit quantities. In the absence � of nctunl operating or laboratory data, it still remains a matter of taking these fac- n rors for the community in question and comparing them against other communities � �chere grit �•olumes are kno��n. � As an erample of the ��ide range oE values encauntered, ��ntrrecordse son enof � be made to eaisting p• �ehich claim as little as 2.5 m'/10° m' oE o�castewater (0.33 tti Et of grit/mil gal) as an a�•erage, others recording an average of qunntity oE as much as li6 m'/10" m' (2-� cu ftlm�� gal). These are a��erages, and do not reflect quantities of grit collected dur- 145