SURFACE AND UNDERGROUND WATERS PROTECTION AGAINST CONTAMINANTS IN STORM SEWAGE FROM URBAN AREAS

Stanisław Krzanowski, Andrzej Wałęga

ABSTRACT

Solutions for storm sewage discharge from urban areas and waste waters management are presented in the paper. Contaminants carried by fall runoff from drained sites such as roads, car parks, squares, etc. pose a real threat to the natural water environment. The literature data gathered by many researchers has proven the maximum contaminants concentration in the storm sewage to occur in the first phase of the fall that tend to last from 15 to 20 minutes. During that phase observed contaminants concentration, which in general tend to consist of total slurry, COD, and chlorides in thaw periods, can be the order of highly concentrated industrial waste, therefore pre-treatment of storm sewage prior to its discharge into the collectors seems unquestionable. The most favourable and proper ways of storm sewage treatment shall be simple, semi-natural retention and infiltration systems, where processes leading to contaminants reduction are very much alike the ones in the natural water-soil environm

Key words: storm sewage, thaw, first runoff wave, sedimentation, infiltration.

INTRODUCTION

Storm runoff waters discharged from drained surfaces such as streets, parking sites, squares, walkways, etc. are 'enhanced' in various chemical substances. Processes that typically contribute to such 'enhancement' are at large caused by air polluted with all sorts of industrial dust or elements of car fumes, surface contaminated with tire and road surface abrasion byproducts, as well as due to improper transport of loose and liquid materials. Another considerable source of pollution are car crashes, mainly due to the hazardous substance spilt. The spilt substances, unless neutralised and disposed of in time, can lead to water system environment contamination. Respecting the regulation of Ministry for Environment dated 29 November 2002 [24] shall provide full protection for surface and ground waters against contaminants originating from sewage or runoff waters if the conditions specified thereof for sewage prior to being discharged to collectors are met. According to clause 20, p.1 of the Regulation stormsewage can be discharged either to surface waters or to soil provided the concentration is not higher than 100 mg·dm^-3 and 15 mg·dm^-3 for slurries and oil derivative substances, respectively.

Water environment can be successfully prevented from being contaminated by the stormsewage by applying simple, semi-natural retention and infiltration facilities, where self-purification processes are very much alike the ones occurring in natural water-land environment [5,16, 28]. Even temporary storage of storm sewage in retention and retention-infiltration reservoirs can both help to maintain a reasonable capacity of sewage system at times of high rainfall, and average and reduce the level of contaminants it contains.

QUALITATIVE CHARACTERISTICS OF STORM SEWAGE

High contaminants concentrations occurring periodically in storm sewage and its adverse effect on collectors has generated the need to solve water protection problem in urban areas in a practical manner. Storm sewage is specific with regards to great variability of periodic and short lasting occurrences of significant volumes of water and contaminant loads to discharge.

The type of contaminants storm sewage contains is related to the management type of the considered catchment, while their amount depends both on the accumulation time and the intensity of flushing, which in turn is a function of intensity, duration and level of a rainfall [4, 20, 22]. Table 1 presents values for selected contaminants concentration in storm runoff recorded by Polish researchers.

The values presented there show great variability of storm runoff concentrations for indices contaminants. Nevertheless, the major contribution to storm sewage contamination comes from total slurry and COD. For a thaw runoff higher contaminants concentrations are noted, especially for chlorides, when compared to a rainfall runoff. It results from snow capacities to accumulate contaminants, especially when it was laid off on the kerbs for long [15,19,20]. Total slurry, whose significant absorption surface for mineral fraction varies between 60-80% of total slurry amount, is more susceptible to other contaminants that major part concerns COD, heavy metals, fats, mineral oils, and phosphates [10, 22]. Significant amounts of such hard to decompose, toxic and mineral substances by no means facilitate the self-purifying processed in waters. Thus, they shall be at first treated mechanically that would result in reducing [17] total slurry and other related contaminants. Higher concentrations of bi ogenic compounds (see Table 1) have been observed sporadically. As it has been proven by the research carried out by Nowakowska-Błaszczyk and Zakrzewski [14], the occurrence of biogenic contaminants can be contributed to runoff that collects area-typical contaminants from green areas. The same researchers found roof runoff to carry the lowest contamination load unless they came roofs covered by roofing paper and galvanised iron (zinc coated sheet). Concentration for total slurry, phosphates, BOD₅, COD and lead varied between 28-35 mg·dm^-3, 5-9.8 mgO₂·dm^-3,: 48-200 mgO₂·dm^-3, and 0.1-0.15 mg·dm^-3, respectively, while it changed within trace amounts limits for phosphates. The detected levels allow to discharge the runoff directly to the collectors.

The maximum of concentration peak for indices contaminants concentrations in the first runoff wave, usually timed at 15-20 minutes of the rainfall (see Fig. 1) is a typical storm sewage characteristics, especially when small catchments are concerned of the area smaller than 50 ha [4,19]. The phenomenon caused real trouble when determining discharge and treatment equipment capacities for storm sewage. That is most likely the reason for recommending by many researchers [25] to construct retention facilities of capacity 100 m³, or even 200 m³ per one hectare of the tight catchment area.

SOLUTIONS APPLIED IN STORM SEWAGE TREATMENT

Increasing costs of traditional discharging storm sewage, related to urban development, as well as its adverse effect both on the drained area and the collector have born the necessity of its local management. The most current issue to focus on can be finding alternative solutions for effective storm sewage management at the site it occurs.

Rational protection of the water system environment against the adverse effect of storm sewage consist in applying easily constructible and operation facilities that perform retention, purifying, and discharge functions. Ponds, retention and retention-infiltration reservoirs, infiltration ditches and permeable surfaces can serve as exemplary facilities. Open retention-infiltration facilities can be also employed for recreational or urban landscaping purposes [21].

STORM SEWAGE RETENTION

Retention is defined [2] in terms of storing storm sewage over specific periods within the system covering both the catchment area and the existing sewage system including sewage treatment facilities or plants.

The contaminants present in the storm sewage, mainly total slurry, are reduced in retention equipment due to sedimentation process. Sedimentation studies performed by Zawilski [27] showed that to achieve low total slurry concentration storm sewage shall be stored in the retention facility over a considerable period (see Fig.2). The time required depends both on the rainfall intensity and its duration.

Nevertheless, even prolonged sedimentation periods do not eliminate all the total slurry from storm sewage, since certain amount of hardly settling total slurry remain to be detected (see Fig. 3 for details).

As illustrated in Fig. 3 the total slurry reduction percentage in storm sewage after 120 min of sedimentation tended to vary between 60-75%, depending on the initial concentration factor. Similar values were obtained by Ślusarczyk and Zawilski [26] who exposed storm sewage to 24 hours sedimentation in the sedimentation column. A significant correlation between total slurry and COD concentrations was found in studies on storm sewage sedimentation. For different sewage type - storm or thaw sewage - the correlation coefficient varied from 0.76 to 0.96 [27, home research]. The substances affecting the COD index were detected in the fine slurry, which in turn was found to be correlated with heavy metal content, whose correlation coefficient according to Palmgren [23] varied between 0.85 and 0.99.

The presented results indicate that sedimentation process is not effective enough to eliminate contaminants to the level compliant to the regulation in question. Due to time constraints of the sedimentation process, prolonged storage of storm sewage would be required, which in consequence would lead to oversized settling ponds, a solution that is not practicable unless vast surfaces are available, which is rarely the case.

Consequently, sedimentation process shall be applied for storm sewage as a pre-treatment technique. Retention ponds and reservoirs are often built for such purposes. A detailed characteristics for such facilities can be found in papers by Edel [3], Fidala-Szope [4], Geiger and Dreiseitl [5], Mioduszewski [12, 13], or Osmulska-Mróz [18].

The main goal of retention ponds and reservoirs is to reduce both the peak outflow of storm waters and the level of contamination. A typical retention collector scheme is shown in Fig. 4. Such reservoirs can be either permanently filled with waters or stay empty over rainless periods. They shall be automatically operated and equipped in emergency overflows rainfalls much less likely to occur than the ones taken as input data for calculations. Grass-like sowing [13]over the reservoir bottom and slopes that would provide a biological filter is recommended in order to improve contaminants biodegradation processes taking place within. Due to slurry content in storm runoffs that may quickly may silt the bowl of reservoir, it is also recommended to apply preliminary settling ponds.

For smaller catchments instead of retention tanks retention ponds can be used (Fig.5). Such ponds shall be wedge -shaped, narrow at the inlet opening and wide at the outlet one.

The part permanently filled with water shall be at least of capacity of 125 m³ per ha of the tight surface. An average depth for such ponds shall fall into the range from 0.9 to 1.8 m, with the inlet part shall be shallower than the outlet one.

The results for reduced contamination percentages in storm sewage obtained in Cracow where a facility at Bieżanów that consists of two reduction chambers and a retention-infiltration pond was studied for total slurry , COD, BOD_5, chlorides, ammonia nitrogen and phosphates recorded the values of 60%, 15%, 30%, 69%, 43%, and 27% , respectively [10]. Similar studies on a retention-infiltration reservoir for storm sewage from Lublin Agricultural and Horticulture Giełda in Elizówka were performed by Zubala [29]. The sampled sewage collected in settling pond and retention reservoir was analysed and the resulted percentage of reduction for total slurry , chlorides, ammonia nitrogen and phosphates was found to be equal to 90%, 40%, 50 %, and 7.4 %, respectively. Similar result have been also reported by Martin [11] who performed his research on regional storm sewage treatment plant in Ontario, Florida. His facility consisted of two ponds in a serial arrangement, namely the r etention one and another with marsh vegetation. Reduction percentages of 60%, 35%, and 40% were recorded for total slurry , nitrogen and phosphates, respectively. All the results confirmed the positive role the retention plays in storm sewage treatment collected from urban areas, with special regards to Z reduction. Marsh type of swamplands helps to reduce biogenic compounds.

STROM SEWAGE INFILTRATION

Infiltration to soil is both the most promising method of storm sewage treatment and in addition a highly effective method of local retention. Soil environment provides a natural filter for contaminants present in storm sewage [5]. Physical, chemical and biological processes into which storm sewage contaminants are involved lead to their significant reduction. It shall be remembered, though, that not all soil types are suitable for storm sewage cleaners. Underground waters covered by a soil layer of a filtration coefficient k<10^-3 m·s^-1 are naturally protected from adverse effects of sewage infiltration into the soil [1, 3, 20]. Rainfall runoff from tight surfaces prior to being discharged to ground need pre-treatment to avoid likely colmatation of bed, apart from runoffs from roof surfaces.

Infiltration facilities for storm sewage treatment can be divided into two system types [4]:

• surface infiltration, where percolation goes through lawns, flower beds, green areas, grass ditches, walkways made of slabs or profiles cubes, roads, parking places and squares covered by a layer of asphaltic concrete, or

• underground infiltration where underground ditches filled with gravel and supporting drainage material or drain wells working either as individual facilities or in a drain galleries arrangement are employed.

Both system types are often interconnected. The minimum distance from the bottom of the dripping layer/ percolator to the maximum level of groundwater table shall not exceed 1.0 m. Infiltration reaches the maximum intensity in reducing the contaminants at the uppermost 30 cm soil horizon [5].

The simplest way to limit the quick run-off is to apply permeable surfaces [3,5]. When in addition they are grass covered the uppermost horizon of soil is especially effective in purifying the storm sewage. Nevertheless, the most typical and widely used storm sewage percolators are open infiltration reservoirs/collectors (see Fig.6).

Water flowing from tight surfaces to retention-infiltration collector, and then to groundwaters, is striped off its negative characteristics due to sedimentation, mechanical filtration, chemical and microbiological processes occurring both at the collector surface and in the filtering layer, and last but not least due to being dissolved in the groundwater [6]. The factor on which efficiency of infiltration processes in eliminating the contaminants heavily depend, is the infiltration time (Fig. 7 and 8). The longer sewage infiltrates the filter bed, the greater reduction in bio-contaminants and solids content will occur [10].

Cwgr_BOD5 –BOD₅ concentration in groundwater, Cwgr_COD - COD concentration in groundwater,

Czb_BOD5 –BOD₅ concentration in collector water, Czb_COD - COD concentration collector water

Cwgr_TS – total slurry concentration in groundwater, Cwgr_zm – mineral slurry concentration in groundwater, Czb_TS – total slurry concentration in collector water, Czb_zm – mineral slurry concentration in collector water, t_d – infiltration time in 24-hour units

Vegetation covering bottoms and slopes of infiltration collectors plays also a positive role in storm sewage purification processes. Sod capacities for retaining petroleum derivatives, biogenic contaminants and slurries are widely known [7]. Biofilters have become widely applied as pre-treatment facilities for rainfall waters [12,13]. Biofilters are shallow collectors of typical depth between 0.3 – 0.6 m at the surface of which vegetation develops eagerly (Fig.9). Solids sedimentation, nitrogen denitrification and nutrients consumption by plants facilitate storm sewage purification processes. Biofilters an be also applied for preliminary treatment of storm sewage prior to their introduction to actual infiltration facilities.

Infiltration processes can also occur in infiltration ditches filled with coarse-grained material [5, 9, 18]. The purifying processes there are yet limited, due to lesser contact between infiltering runoffs and live soil layers. Thus, in such systems pre-treatment is absolutely necessary before they are introduced to underground infiltration facilities.

The degree by which the organic contaminants can be reduced for the combined retention and infiltration facilities is significant and varies for BOD₅ and COD between 30-60%, whereas an increase in total and mineral slurry as well as biogenic compound content was recorded in the filtrate [10]. Their content was over-doubled and reached 15-30%, respectively. The observed increase in slurry and biogenic compounds content could have been caused by minerals flushing from bottoms and the bed, lack of developed vegetation at the collector bottom and short infiltration times. Research by Osmulska-Mróz and Sadkowski [15] has proven a positive influence of graminoids upon the contaminants reduction capacity in storm sewage to be true. In their studies on storm sewage infiltration in grassy ditches the total slurry, COD, and ether extract were reduced to the values ranging between 41-94%, 30-91% and 50-93 %, respectively. The levels for reduction reported by Oberts [21] were similar to the ones obtained by Osmulska-Mróz and Sadkowski, and varied between 17-90% and around 50% for total slurry and biogens, respectively. His research was conducted on infiltration reservoirs grown over with graminoids.

The presented results clearly indicate a positive role of infiltration in the process of reducing storm sewage contaminants. Storm sewage treatment proved significantly more effective on surfaces overgrown with graminoids when compared to surfaces on which biofilter has not developed.

SUMMARY

The contaminants present in storm and thaw sewage, mainly such as total slurry, COD, chlorides, heavy metals and petroleum derivatives substances, poses a serious threat to water system and environment. The heaviest contaminants concentrations are observed at the first lasting 15 to 20 minutes fall phase. That is so called first runoff wave for which recorded concentrations can be relevant to heavily contaminated industrial waste, thus the pre-treatment necessity for such waters before their introduction to the collector cannot be questioned. Retention and infiltration of storm sewage under natural conditions provides an effective tool for limiting its adverse influence on the water systems. Therefore, while designing drainage systems the storm sewage shall be collected at the earliest site possible, stored in either natural or artificial reservoirs and enabled to percolate into the ground. Due to the combined co-influence of physical, chemical and biological processes the contaminants lev el can be reduced. It should be reminded also, that re-introducing storm sewage waters into the soil allows us to increase water systems capacities, which consequently contributes to limiting the extreme water levels and flows in surface collectors.

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