| Front page | | Contents | | Previous | | Next |
Udsivning af spildevand fra afløbssystemer
Leakages in sewers may result in infiltration of groundwater as well as exfiltration of
wastewater. A significant amount of information is available on infiltration of
groundwater into sewers. However, only a limited number of studies have been made on
exfiltration of wastewater and the consequence hereof upon the groundwater quality.
A few studies attempt to quantify the pollution of the urban groundwater, however, they
suffer from difficulties in distinguishing between pollution from sewers and pollution
from other sources, e.g. industrial areas. Some studies attempt to quantify the
exfiltration during normal operational conditions. They are either based on indirect
determination of the exfiltration, or based on flow measurements in the sewer by
application of tracers. Common for these studies is that they show significant
exfiltration from urban sewers: up to 25% of the wastewater may exfiltrate into the
groundwater before the wastewater has left the catchment. In intercepting sewers, average
exfiltration rates of 3 l s-1 km-1 have been determined. The
indirect methods for determination of the magnitude of the exfiltration are assessed to be
based on such large a number of assumptions that they are considered non-conclusive. The
direct determinations are based on measurement of very small differences in flow and are
consequently difficult to perform with precision. Even though direct measurements
generally are "better" than indirect determinations, they too must be used with
great care, due to the problems associated with precise flow measurement under field
conditions.
The literature studies agree on TV-inspections being non-conclusive when identifying if
– and to which degree – wastewater is exfiltrated from sewers. In other words:
not all damages of a sewer give rise to exfiltration when the groundwater table is located
below the sewer. The goodness of this conclusion is difficult to assess because the
missing correlation between TV-inspection of leakages and exfiltration also could be due
to difficulties in correctly determining the exfiltrating wastewater.
The present project highlights studies of those conditions, which control the
exfiltration from leakages in sewers as well as the resulting flow and effects of the
exfiltrating wastewater on soil and groundwater. This is done investigating and
quantifying the exfiltration over time with focus on water and pollutant transport. Soil
characteristics, the type and size of the leakage as well as the operation of the sewer
are taken into account. The study is based on a literature review and a pilot scale study.
Furthermore, a field investigation has been made of an identified leakage in a sewer in
Aalborg, where the sewer has been excavated and the surrounding soil analyzed. The results
hereof have been compared to the literature study and the pilot scale experiments.
The pilot scale experiments were performed by circulating fresh wastewater through a
sewer pipe situated in sand. The pipe contained well-defined leakages from which the
wastewater could exfiltrate into the soil and consequently be collected. The pilot scale
investigations were made under realistic conditions. For example, the soil temperature was
typical for Danish soils and water velocities and sheer stresses in the pipe at or above
typical Danish values.
The investigation has shown that the exfiltration from sewers under constant flow
conditions is reduced with time and asymptotically reaching a constant level after 1-2
days. This level is independent of the type of sand in which the pipe has been situated,
however, determined by a semi permeable zone surrounding the leakage. In accordance with
basic hydraulic theory on exfiltration through a semi permeable layer, proportionality
between the exfiltration and the water level above the leak as well as proportionality
between the exfiltration and the surface area of the leak was found. Procedures for
calculating the risk of leakage have been established in agreement with these findings.
It is furthermore documented that conditions in a pipe corresponding to a storm event
results in an increase in the exfiltration. The same is the case after cleansing of the
sewer or when the water level is changed from above the sewer to below the sewer, i.e. at
alternating infiltration/exfiltration conditions. All these increases in exfiltration are
reversible as the exfiltration rate decreases rapidly when the cause of the increased
exfiltration is removed. After 1-2 days, the exfiltration is back at the initial, low
level.
The magnitude of the exfiltration depends on the type of leak: an open joint or a leak
in the bottom of the sewer. The exfiltration rate per unit area is largest in the later
case, where the permeability of the semi permeable layer is of the same magnitude as known
from clogging biological sandfilters.
Pollutants and microorganisms are transported out of a sewer with the exfiltrating
wastewater. The pilot study shows that a significant reduction of pollutant content and
microorganisms takes place in the vicinity of the leakage. E.g. the bacteria investigated
are removed almost completely. The investigations indicate that DEHP and anionic
detergents also are reduced significantly, whereas no conclusion can be drawn on other
micropollutants investigated as these were only found in small concentrations in the
wastewater. Nitrogen was found on the form of nitrate because nitrification takes place in
the aerobic soil beneath the leakage and nitrate is consequently discharged into the
groundwater. Similarly it must be assumed that soluble, slowly degradable micropollutants
– to the extent to which they are present in the wastewater – will behave
similar. In case the soil contains macro-pores, a higher pollutant transport than found in
this study should be expected. However, a sand layer around a pipe is expected to reduce
the transport in the macro-pores.
The field study – where a sewer was excavated – confirms the results of the
pilot scale studies: leakages in sewers are clogging due to biofilm growth and fine
particles. After many years of exfiltration, remains of wastewater were found directly
beneath the leak, however, already half a meter below the leak, no wastewater compounds
were found.
Based on the present study a risk assessment for groundwater pollution from leaky
sewers can be made based on assessment of the number, type and extent of damages. It is
suggested that TV-inspections be used as a tool to assess such damages. As not all damages
are leaks, this method will overestimate the leakage. Together with the hydraulic
conditions in the sewer, this information allows the determination of a maximum
exfiltration rate. A leakage factor is introduced for this purpose based on the results of
the present study. Taking the natural formation of groundwater into account together with
the retention and/or degradation of a specific pollutant, an assessment of the maximum
effect on the groundwater can be made.
Comparison of such risk assessment based on the result of the present study with prior
investigations from the literature shows markedly less exfiltration. Even when the sewers
in the catchment are in poor condition, only a few percent of the total wastewater
production will exfiltrate into the surrounding soil. Such risk assessment takes into
account the average exfiltration from a sewer and does not cover phenomena like special
soil conditions in combination with a groundwater well in the ultimate vicinity of a sewer
in poor shape. To assess such concrete risk, the hydrogeological conditions, the operation
of the sewer, the location of the leeks relative to the groundwater well and the size and
type of the leaks must be taken into account.
| Front page | | Contents | | Previous | | Next | | Top
|