328
D.H. Waller"
The composition of combined sewage is highly variable. Most of the variations in
samples collected from the Halifax system could be explained in terms of the rate of
combined sewage flow and the time, after the beginning of the storm, at which the
sample was collected. Analysis of the data in terms of these parameters showed that solids
concentrations increased with the rate of flow. This increase of combined sewage solids
concentrations with increasing flow rate was in part a reflection of a similar effect that
was apparent for concentrations of surface runoff solids, and partly due to the scour of
deposited solids from sewers. Bacterial densities in combined sewage samples decreased as
the rate of flow increased, reflecting the dilution of sanitary sewage by relatively less
contaminated surface runoff. Concentrations of all constituents tended to decrease as a
storm progressed. If, however, the rate of flow increased late in a storm, solids
concentrations increased accordingly.
A representative concentration of a combined sewage constituent would have to
account for variations in composition due to flow rate and time during storms, and would
have to be flow-weighted to represent the wide variations that occur in rates of combined
sewage flow. Information obtained for the Halifax area was not sufficiently complete to
permit calculation of a flow-weighted average concentration that would represent
variations in every storm that occurred during the sampling period. Complete flow data
and information about variations in combined sewage composition were available for a
number of individual storm events, however, and solids concentrations and flow rates for
two major storms in 1970 were used to calculate rates of solids discharge from the
Halifax study area. A 3.89 inch rainfall in August of 1970 was associated with the
discharge of 6970 pounds of combined sewage suspended solids and 2690 pounds of
volatile suspended solids. A 2.74 inch rainfall in September of 1970 resulted in the
discharge of 5270 pounds of suspended solids and 1990 pounds of volatile suspended
solids. The average amount of solids discharge in the two storms is compared in Table 2
with solids discharge values obtained in other North American studies. All of the
information in Table 2 is expressed as a rate of solids discharge in pounds/acre/year; this
rate is the product of the amount discharged in the recorded storms, times the ratio of
the rainfall rate (in inches/year) to the storm rainfall. For the Halifax data the rainfall
rate during the sampling period coincides with the annual rainfall. For San Francisco and
Detroit, annual rates have been used. The fact that the Buffalo data is consistent when
the rainfall rate during the sampling period is used, strongly indicates that solids
accumulation is a function of time and similar for many urban areas. When all of the data
is expressed in this manner, rates of solids discharge for the Halifax drainage area are
shown to be comparable with those from other communities having different hydrologic
and geographic characteristics, and the order of magnitude of combined sewage pollution
loads for these communities appears to be clearly defined.
The Halifax data (and much of the other data) in Table 2 represents rates of solids
discharge for individual storms. Examination of information obtained in other storms on
the Halifax drainage area indicates that these rates are reasonably representative of rates
of solids discharge for the drainage area during the summer months. These rates are not
intended to represent the total annual discharge of solids discharge for the drainage area.
An understanding of the sources of pollutants in combined sewage can contribute to
the development of more effective measures for pollution control. The most significant
contributions to the composition of combined sewage are sanitary sewage, surface runoff,
D.H. Waller"
The composition of combined sewage is highly variable. Most of the variations in
samples collected from the Halifax system could be explained in terms of the rate of
combined sewage flow and the time, after the beginning of the storm, at which the
sample was collected. Analysis of the data in terms of these parameters showed that solids
concentrations increased with the rate of flow. This increase of combined sewage solids
concentrations with increasing flow rate was in part a reflection of a similar effect that
was apparent for concentrations of surface runoff solids, and partly due to the scour of
deposited solids from sewers. Bacterial densities in combined sewage samples decreased as
the rate of flow increased, reflecting the dilution of sanitary sewage by relatively less
contaminated surface runoff. Concentrations of all constituents tended to decrease as a
storm progressed. If, however, the rate of flow increased late in a storm, solids
concentrations increased accordingly.
A representative concentration of a combined sewage constituent would have to
account for variations in composition due to flow rate and time during storms, and would
have to be flow-weighted to represent the wide variations that occur in rates of combined
sewage flow. Information obtained for the Halifax area was not sufficiently complete to
permit calculation of a flow-weighted average concentration that would represent
variations in every storm that occurred during the sampling period. Complete flow data
and information about variations in combined sewage composition were available for a
number of individual storm events, however, and solids concentrations and flow rates for
two major storms in 1970 were used to calculate rates of solids discharge from the
Halifax study area. A 3.89 inch rainfall in August of 1970 was associated with the
discharge of 6970 pounds of combined sewage suspended solids and 2690 pounds of
volatile suspended solids. A 2.74 inch rainfall in September of 1970 resulted in the
discharge of 5270 pounds of suspended solids and 1990 pounds of volatile suspended
solids. The average amount of solids discharge in the two storms is compared in Table 2
with solids discharge values obtained in other North American studies. All of the
information in Table 2 is expressed as a rate of solids discharge in pounds/acre/year; this
rate is the product of the amount discharged in the recorded storms, times the ratio of
the rainfall rate (in inches/year) to the storm rainfall. For the Halifax data the rainfall
rate during the sampling period coincides with the annual rainfall. For San Francisco and
Detroit, annual rates have been used. The fact that the Buffalo data is consistent when
the rainfall rate during the sampling period is used, strongly indicates that solids
accumulation is a function of time and similar for many urban areas. When all of the data
is expressed in this manner, rates of solids discharge for the Halifax drainage area are
shown to be comparable with those from other communities having different hydrologic
and geographic characteristics, and the order of magnitude of combined sewage pollution
loads for these communities appears to be clearly defined.
The Halifax data (and much of the other data) in Table 2 represents rates of solids
discharge for individual storms. Examination of information obtained in other storms on
the Halifax drainage area indicates that these rates are reasonably representative of rates
of solids discharge for the drainage area during the summer months. These rates are not
intended to represent the total annual discharge of solids discharge for the drainage area.
An understanding of the sources of pollutants in combined sewage can contribute to
the development of more effective measures for pollution control. The most significant
contributions to the composition of combined sewage are sanitary sewage, surface runoff,