3. Development of functional elements
3.2. Dispersion term
3.2.2. Dispersion in surface water
In the migration to surface waters, two principal routes with substantial differences in the consequences of the release can be defined: a route with a wastewater treatment facility and a route without one. In addition to this fundamental division, there may be other slighter differences between the routes, e.g. in relation to physical or chemical pretreatment devices. These devices are taken into account in the assessment of the release quantity. Furthermore, the recipient may be river, lake or sea. The surface waters dispersion model consists of four submodels (Fig. 10).
Determination of damages
·�
Lake or sea basin system
-
submodell
Surface River system submodel s I !ck
submode I
l
- ,.
wastewater treQtment system submode I
�
Quantity assessment
Fig. 10. The main components of the surface waters dispersion model.
Activated sludge treatment is the most frequently applied wastewater purification method in the Finnish pulp and paper industry. A typical facility consists of primary clarification basin(s), aeration basin(s) and secondary clarifying basin(s) (Fig. 11). Quite often an activated sludge facility is also furnished with an equalization basin and an emergency basin where the wastewater can be directed in the case of a detrimental release. Though not considered here, the function of the emergency basin can be taken into account in the probability term of the index.
Because it is the aeration basin(s) where the concentration of a released deleterious material is of interest, also the secondary clarification basins can be omitted if
pass-through effects are not considered. The model does not account for sludge recirculation, which speed up mixing the released material from aeration basin to secondary clarifier. Therefore, omitting the effect of sludge recirculation leads to a modest overestimation of aeration basin concentration. Only the highest concentration in
each aeration basin is recorded. This dilution calculation procedure is valid also for most other wastewater treatment systems.
H2 504 ----,
Ga(OH)2 lnfluent r-tJtrTents
I I I
'----0---'1
I
i - - - -
-0 --- - _,
.,
<)a,
I.. '
7 dawatQl'"OO sludge
1. Prlm!lry cl!ll"lfler 2. Neutral rzatlon bas In 3. l:QI.Jlll rzatlon basin 4. Aeration basin 5. Secondary clarlfler 6, Stab! llzatlon basin 7. Sludge de-...terfng
Fig. t t. A schematic description of an activated sludge treatment facility used in dilution calculations.
The single basin dilution of an activated sludge treatment facility is calculated using the equation:
where ci
= max {cik}
= maximum concentration in basin i (mg/I)
= concentration in basin i at time k · t (mg/1)
= influent concentration.to basin i at time k · �t (mg/I)
= wastewater flow (m3/h)
(9) (10)
= effective volume of a basin i (m3)
= time step (h)
The effluent concentration in a basin at any step is used as influent concentration in following basin. The iteration is continued until the concentration in the critical basin has passed its maximum. As was described above, the effects on a wastewater treatment plant are calculated from contaminant concentration with no reservation on substrate concentration. Volskay & Grady (1988) have presented that this approach is valid when the inhibitor behaves in a noncompetitive way. Because of the scarcity of data, noncompetitive behaviour is generally accepted in this model.
Transport and dispersion of a release in a river system has often been approximated using a one-dimensional equation (van Genuchten 1981):
oclot
where
X
DL u
s
= distance in direction x
= longitudinal dispersion coefficient
= flow velocity in direction x
= concentration rate of change caused by nonhydraulic physical, chemical and biological processes.
(11)
Hypothetical applications of this one-dimensional river model have been described by e.g. Kontaxis & Nusser (1982) and Kyla-Harakka-Ruonala (1989).
A short-duration release causes a time-variable distribution of concentration at different points of a waterway. The time related effective concentration to which a target population is exposed constitutes the exposure. The exposure is defined as the integral of concentration over time:
E
where E c(t)
= J c(t)dt (12)
= exposure
= time-dependent concentration.
If the effect of the exposure is presumed to be independent of the time related concentration profile, a simple practice of calculating an average concentration could be applied when determining the effects. French & French (1989) have analyzed a large volume of test data and demonstrated that the use of an average concentration is not valid.
In order to simplify the calculations, only the maximum values of spatial concentration distributions are utilized in this index system. Excluding the variability of concentration with time has no significant effect on the results within a single recipient. The differences between the recipients may be substantial if the retention time is very low in some recipient. In these cases, the error due to the concentration's steep time-dependence must be treated explicitly.
Using the equation (10) in calculating the dilution in the basins of the activated sludge facility is justified, because the mixing is efficient. Although this is seldom true of a lake (or sea) basin system, the dispersion calculation for a lake basin is made using the equation (10) with slight modifications:
(13)
= contaminated inflow to basin i (m3/h)
= uncontaminated inflow to basin i (m3/h).
The problem of partial mixing is handled by incorporating a subjective element in the definition of the basins. The lake is divided into consequent basins and, when necessa.•7, only a portion of the water volume is considered to involve in diluting the release. The shape and number of the basins should be in accordance with the migration and mixing of wastewaters during a normal emission situation, with the exception that the specific gravity of the accidental release may differ from that of wastewater. The iteration is continued until the concentration in the basin begins to decline. The maximum concentration in each basin is recorded.