e:> Pergamon

PIT: S0273-1223(99)00012-8

Wat. Sci. Tech. Vol. 39. No.2. pp. 93-IOt. 1999.C 1999IAWQ

Published by Elsevier Science LtdPrintedin Great Britain. All rights reserved

0273-1223/99 $19'00 + 0110

STORMWATER AND LAMELLASETTLERS: EFFICIENCY AND REALITY

A. Daligault*, D. Meaudre*, D. Arnault**, V. Duc**,N. Bardin**, N. Aires***, D. Biaut, 1. Schmidt,P. Clementz and 1.-Y. Viau:j:

• LaboratoireRegionalde l'OuestParisien, 12 rue Teisserene de Bort,78190 Trappes, France•• S.I.A.R.V.S.G.• l7 rue GustaveEiffel,91230Montgeron, France••• Agencede l'Eau Seine Normandie, 51 rueSalvadorAllende,92027Nanterre Cedex; Francet DirectionDepartementale de l'Equipement de l'Essonne, Routede Lisses,91100 Yillabe, France:t I.S.D. Environnement (Environnement resultsofthefusion ofltera-SeparepurandSaint-Dizler), 21, Rue EllseeReclus, 59650 Villeneuve D'ascq, France

SUMMARY

Two senling devices treating rainwater from urban drainage areas and equipped with secondary sludgetreatment were monitored over a period exceeding one year. The two devices differ in their design and in thequantity and quality of effluents received. Full analyses over approximately 30 mean samples of the intakeand discharge effluents allowed judgment of the mean efficiency of pollution control and the reduction inimpact of the storm water. The observed settlement was compared to measurements of settling velocityduring several rainfalls. From analyses of the settled or extracted sludge it was possible to characterize thepollutants retained in the sludge . Particular attention was paid to the function ing and operation of the devices.~ 1999 IAWQ Published by Elsev ier Science Ltd. All rights reserved

KEYWORDS

Particle settling devices; sludge extraction; settling velocity measurements; facility operation; pollutioncontrol performance.

INTRODUcnON

The reduction of pollution relating to urban runoff is a major concern for the Syndicat Intercommunal of theVilleneuve Saint-Georges region. the Agence de l'Eau Seine-Normandie and the Direction Departementalede I'Equipement of Essonne and requires the installation of treatment facilities. Many techniques arecurrently available to combat the urban wet weather pollution. Because most of this pollution comes inparticle form, settlement is the process most widely used at the present time.

Because of land scarcity and the large number of outlets, the S.I.A.R.V.S.G. opted for two compact.prefabricated facilities. To facilitate operation, these plate and tube settling devices were equipped with anexperimental system to extract automatically and treat the sludge obtained by sedimentation. The objective

93

94 A. DALIGAULT tt al.

is to remove sludge from the standard section of the facility to avoid disturbing and re-circulatingsedimented particles into the main body of effluent. The device also enables concentrating the sludge tofacilitate their removal and reduce maintenance frequency. Monitoring of the two facilities lasted for over ayear.

DESCRIPTION OF THE DEVICES AND PARAMETERS MEASURED

DLRS210 deyice in BrunQY

The approximate surface area of the drainage area of the Brie road in Brunoy is 6.7 acres (2.7 hal comprisingeducational and sporting infrastructures (55%), apartment buildings (36%) and roadways (8%). The "active"area measures 3.5 acres (1.4 hal.

The facility is designed to accept a 210 Us nominal flow rate, approximately the maximum flow that occurswith ten-year frequency. The flow entering the settling device is regulated by a storm water overflow ofswirling static separator type.

The plate settling device DLRS210 (Figure I) built by SEPAREPUR (ISD) comprises:

a compartment with a screen (19 mm intervals between bars);a lamella cross-flow compartment presenting a 4.8 mlhr surface load at nominal flow rate;a siphon outlet equipped with an automatic seal.

The sedimented sludge collects in a cone situated under the lamella settling blocks. From this cone (whichhas walls sloping at a 35° angle) the sludge are pumped at a rate of 3 Us into a 20 m3 sludge silo functioningby simple sedimentation, the overflow returning upstream to the screen compartment. Pumping frequency is10 minutes per hour.

DP3DL device in Yi~neux.

The drainage area of the Les Bergeries branch pipe in Vigneux covers approximately 259 acres (105 halcomprising private houses (56%), apartment blocks (20%), shops (3%), industrial estates (8%), a park (4%)and roadways (9%). Two storage reservoirs reduce the drainage area to 190 acres (77 ha) with approximately9.5 acres (20 ha) "active" surface. The I to 1.5 litres per second dry weather flow rate represents a 30 to 40inhabitant-equivalents effluent flow.

The treatment facility is sized for a maximum inflow rate of 560 Us corresponding to the flow over a six­month period. The effluent entering the facility is limited by a mobile flow controller.

The cellular DP3DL56O settling device (Figure 2) built by SAINT DIZIER (lSD) comprises:

a compartment with a screen (10 mm between bars);a cellular cross-flow compartment consisting of PVC honeycombed blocks and presenting a 7.2 m/hrsurface load at nominal flow rate;a siphon outlet with an automatic seal.

The sludge collected in the cone (walls sloping at 18°) under the plate settling blocks is pumped to asecondary treatment system. The first system studied was a filter basket cIadded with a polyamide filter witha SO um mesh, the filtrate being returned upstream to the inlet of the facility. The second system is athickener, functioning by simple sedimentation, the sludge water being evacuated by a floating skimmerafter 10 hours' sedimentation.

Measurements

Storm water and lamella settlers 95

Instrumentation on the sites. Each drainage area is equipped with a rain gauge with tipping bucket. Flowrate is monitored:

at Brunoy by measuring water height in the rectangular spout fitted on the outlet;at Vigneux, upstream from the storm water overflow, by a Doppler height-velocity flow meter and atthe facility outlet by measuring the water height in a calibrated collector.

Figure I. Aluminium lamellar cells with cross current technology - bar screen.

96 A. DALIGAULT et a/.

Figure 2. 3D lamellar cells with cross current technology.

The flow meters respond proportionally to the volume of flow past automatic samplers located at the facilityinlet and outlet.

Laboratory analyses. Physical and chemical water analyses of mean samples were carried out for 27 rainevents in Brunoy and 29 in Vigneux.

Stormwater and lamella settlers 97

The following parameters were measured: Total Suspended Solids (TSS), COD, CODad2. BOD5. NTK(KN), total P • pH. conductivity, Cd, Cu, Pb, Zn, particle sizes (measured by laser) . Other analyses wereconducted during the former rain events only: turbidity, COT, HAP, Cd-Cr-Ni at Brunoy, Cr-Ni-Zn inVigneux, organic suspended solids and SSad2 in Vigneux.

Additionally, settling velocity of particles from 7 rain events runoff was determined for the entire duration ofprecipitation, according to the principle of uniform suspension (CERGRENE 95 protocol).

Sludge. The sludges extracted and stored in the secondary treatment systems were sampled and analyzedtwice in Brunoy and three times in Vigneux. The same parameters as for runoff water were analyzed. After14 months' operation. the Vigneux settling device was drained. The water and sludge stored in the facilitywere then studied.

ANALYSIS OF RUNOFF WATER

Inflow pollutants

Mean pollutant concentrations indicated in Table 1 correspond to the mean values obtained for each rainfall,weighted by the flow volumes. The annual load was calculated by multiplying measured loads by acorrecting factor: height of usual annual rainfall I height of rainfall during the rain events analyzed (Table 2).

Table I . Extreme and mean concentrations

CONCENTRATIONS Brunov VizneuxMin Max Mean Min Max Mean

TSS (mgll) 11 458 158 25 964 199D(O 5) (mm) 1I 72 30 7 85 37Fraction> 50 JIm (% vol) 12 57 33 4 70 40Conductivity (fi S/cm) 50 244 99 107 402 189pH 6.9 8.2 7.5 6.8 8 7.5

COD (mg/I) 18 299 68 26 561 121

CODad2 (mg/I) 13 157 - 24 121 -BOD (mg/I) 3 29 10 4 168 17BODad2 (rng/l) 3 22 - 3 33 -NTK (KN) (rng/l) I 12. 2.8 I. 50 4.7

tot P (rng/l) 0.3 4.7 0,56 0.3 191 11

Cd (ug/l) 0.3 1.15 0,9 0.1 3,2 10

Cr (ug/l) 0.8 3.5 - 1.5 8.7 2.2

Cu (fig/I) 7 59 23 6 52 24

Ni (ug/l) 1.9 3.9 - <1 1.4 -Pb (JIg/I) 2 210 52 4 404 69

Zn (ug/l) 210 2900 607 30 640 146

Hydrocarbons (mgll) 0.1 4.9 1.2 0.1 17 3.3

COT (mg/l) 5.7 14.3 - 6.6 30 12

HAP (uWl) <0.16 <0.70 - <0.16 <1.59 -

The Vigneux site engenders more pollution than the Brunoy site, especially for the organic parameters. Zincis an exception: concentrations in Brunoy are very high, the highest values being measured during light rainevents, which is the opposite of the other parameters.

98 A. DALIGAULTetal.

Table 2. Annual mean and specific loads

ANNUAL Brunov VizneuxLOADS Total / active ha Total / active haTSS (kg) 1400 1000 26000 1300COD (kg) 600 430 16000 800BOD (kg) 92 66 2300 120NTK(KN) (kg) 24 17 630 32totP (kg) 4.9 35 150 7.3Cd (g) 8 5.7 130 6.4Cr (g) - - 300 ISCu (g) 200 140 3200 160Pb (g) 460 330 9200 460Zn (g) 5350 3800 19000 970Hydrocarbons (kg) 11 7.7 440 22COT (kz) - . 1500 79

Elimination of Total Suspended Solids aSS)

Brunoy. TSS removal range from 0% to 90%, with a 54% mean efficiency. Mean annual concentration is158 mgll at the inlet and 73 mgll at the outlet. The most important removal rates can be explained principallyby the volume of the settling device; indeed, with a storage capacity of approximately 30 to 40 m 3, aconsiderable percentage of the flow out consists of water that has settled over a long period (runoff volumesfrom 12 out of 27 rain events were below the storage capacity).

For small volumes of rainfall, the concentration of suspended solids at the outlet ranges from 10 to 15 mgll.However, when sludge extraction operates regularly (10 minutes per hour), waters from each part of thesettling device homogenizes and the mean concentration of suspended solids at the outlet attains a value of30 mgll .

Table 3. Settling velocities

v

Sedimentation velocities Flow rateTSS V20 V50 V80 peak mean

Brunov !(mwl) Ilm/h) (rn/h) I(m!h) !lVs) tVs)[Ram 16 [Inlet 27 < 0.2 0.2 5 24 5

I IOutlet 16 <0.2 < 0.2 4rgneux

Rain 19 Inlet 75 <0.2 0.4 6 130 36Outlet 61 <02 02 7

Rain 20 Inlet 27 <02 <0.2 3 121 38Outlet 25 <02 <02 2

Rain 21 Inlet 35 <02 < 0.2 I 85 30Rain 22 Inlet 118 <02 0.2 3 185 39Ram 28 Inlet 290 < 0.2 0.8 19 405 93

Outlet 213 < 0.2 02 2Rain 29 Inlet 323 <02 08 3 125 28

Outlet 219 <02 0.2 I

High removal rates are observed when runoff water TSS concentrations exceed 300 mgll: 40 to 90%.Settling process reaches its limits when the amount of effluent suspended solids is very low ($ 30 mgll)which is confirmed by settling velocity measurements: for rain event no. 16, 50% of the suspended solids

Stormwaterand lamcllasettlers 99

has a settling velocity less than 0.2 mlhour (Table 3). For such events, removal rates of suspended solids canonly be low.

VignelU". TSS removal rates ranged from 0 to 60%, except during one event (-33%) which occurred aftero~e ~ear of operation. Th!s evidenced disturbance .and re-circulation of sediment into the facility, which bythis time was saturated with sludge. The 60% maximum removal rate was reached with an atypical effluentwith a particularly high content of organic nitrogen and BODS.

The approximate 30% mean removal rate obtained from water analyses corresponds to a 5.6 t quantity ofsolids trapped in the settling device over a 13 month period. This amount of solids is fully comparable withthe quantity of sludge estimated when drawn off: 7 1. The mean annual concentration is 199 mg/l at the inletand 143 mgIJat the outlet

For high inflow volumes and TSS concentrations (300 to 960 mg/l), removal rates prove extremely variable,ranging from 16 to 60%. However, in any case, the outflow concentration remains relatively high: from 200to 780 mg/!.

Observed removal rates are consistent with the settling velocities measured over 7 rainfalls when TSSconcentrations exceed 50 mg/!. Indeed, settling process is all the less efficient assuspended solids within theeffluent are less suitable to settle. Calculation of removal rates according to settling velocity curves andmean runoff rates overestimates the real efficiency of the settling device. The difference is all the moreimportant as the V50 is lower. The values obtained are well under the ranges quoted by Chebbo (TSM no.II, November 1995).

Remoyal of po!lutants from runoff water

Heavy metals. On both sites, the behavior of lead correlates closely with the behavior of the suspendedsolids, as the lead removal rates prove. Cadmium, Chrome, Copper and Zinc concentrations and removalrates do not correlate with the figures for the suspended solids, thus demonstrating the highly irregularassociation of these four metals with the suspended solids.

In Bruney, the inflow shows high concentrations of zinc. The outflow concentrations are even moreimportant, either due to sedimented zinc returning into the solution, or due to consumption of the sacrificialzinc anode (Table 4). The estimated amount of metals present in the sludge when the Vigneux device wasdrawn off proves that settling metal po!lutant is durably trapped in the sludge.

Table 4. Mean removal rates of heavy metals

MEAN Brunov VigneuxCONCENTRAnONS Inlet Outlet Yield Inlet Outlet YieldCd (Ilg/l) 0.90 0.68 24% 0.96 0.71 26%Cr (J1g11) - . - 2.2 1.2 45%Cu (Ilg/I) 23 14 40% 24 17 29%Ph (Ilgll) 52 29 44% 69 50 28%Zn (U2/1) 607 837 -38% 146 106 27%

Chemical and biochemical demand/or oxygen. For low intensity rain events (S 10 mmlhr over 6 minutes)CODad2 represents 75% and 63% of the inflow total COO at the Brunoy and Vigneux facilities respectively .This explains why COD removal rates are lower than those of suspended solids. For intense rain events,there is a definite reduction in the immediate impact effect, with removal rates ranging from 50 to 75% inBrunoy and 42 to 66% in Vigneux. Mean removal rates of 30% in both cases also hide phenomena of re­solubilisation or biological degradation, evidenced when sludge were drawn off the Vigneux facility: trappedCOD is estimated at half of the COD calculated by deducting the outflow from the inflow loads (we shouldremember that there is a dry weather flow in Vigneux).

100 A. DALIGAULTdal.

BODS mean removal rates are 3 I % in Brunoy and 28% in Vigneux. Removal rates during high intensity rainevents, when mean concentrations exceed 20 mgIJ, are satisfactory yet less than those obtained for COD:from 25 to 67% in Brunoy and up to 46% for Vigneux.

Nitrogen and phosphorus. In Brunoy, many negative removal rates are observed for nitrogen andphosphorus, which can be explained by re-dissolution when the water remains in the settling device for longperiods. This is the case with most of the water present in the discharge effluents , because of the size of thesettling device.

In Vigneux, the small proportion of nitrogen (1/4) and phosphorus (1/3) in the drawn off sludge compared tothe quantities estimated by deducting outflow loads from inflow loads also provides evidence that these twoforms of pollutants are not durably trapped in the sludge at the bottom of the facility.

Total hydrocarbons. In Brunoy, hydrocarbons mean removal rate is 26%. After starting up regular sludgeextraction , the hydrocarbon concentrat ions in the discharge effluent (> I mgIJ) for low volumes of rainfallwere distinctly higher than at the beginning of the study (0. I-0.5 mg/l), This indicates either dissolution ofthe hydrocarbons or re-suspension of the fine particles onto which they are adsorbed.

In Vigneux, the effect of sedimentation on pollution by hydrocarbons is very interesting: a mean removalrate of 37%, and removal rates from 40 to 88% when inflow concentrations exceed 5 mgIJ.

OPERATION OF THE FACILITIES

The screen

We observed that the most important rain events carried a large amount of floating matter, causing thescreen to become clogged. This implies that sites which receive large quantities of floating matter requireautomatically opening gates. In Vigneux, the screen clogging caused load losses and generated earlyoverspill upstream.

Easy access, with a tray to collect the floating matter, as in Brunoy, facilitates cleaning operations.

The sedimentation compartment

From examination of the Vigneux settling device when it was emptied, we drew the following comments:

the 18° slope in the sludge collection cone is not steep enough to allow the sludge to drop into thepump;as the flow carries a lot of plant debris, the honeycomb blocks become clogged, interfering with thesmooth operation of the device and singularly complicating emptying operations. A lamella settlingdevice would no doubt have worked much better;it is essential to close off all space on both sides of the screen or the honeycomb blocks, in order toforce the effluent to flow through the treatment systems.

Secondary treatments

The prototypes studied showed that:

pulsed (5 s) pumping is more efficient than continuous extraction;too slow an extraction rate leads to sludge accumulating and clogging of the pump (as happened bothin Brunoy and Vigneux);too fast an extraction rate (as in Brunoy) causes agitation of the water, encouraging the re-dissolutionand re-suspension of fine particles and pollutants (NTK, Hydrocarbons);the sludge storage volume must be suited to the quantities of sludge to be collected (volume wasinsufficient in Vigneux).

Stormwater and lamellasettlers 101

Analyses of the drained sludge and sludge from the secondary treatment confirm the observations made onthe runoff water:

only small percentages of nitrogen and phosphorus are trapped.the situation for hydrocarbons is more contrasted: retention was good in Vigneux and losses high inthe Brunoy facility.

CONCLUSION

Study of these prototypes provides us with practical data to devise a strategy for controlling rainwaterpollution in urban environments, and to achieve a better match between equipment and sites:

I. Know the receiving waters: its physical and chemical qualities and limits.2. Know the drainage area: its characteristics: active area, peak flow rate, land use; the sewage systems;

and the nature of runoff water: settling velocity, quantity and type of pollutant transported.3. Set objectives for the effluents discharged and types of rain to treat.4. Choose the treatment system so as to optimize capital investment (stormwater overflow + treatment

of the passing flow, or storage +deferred treatment)5. Choose the right dimensions for the facility (in the case of a lamella settlers: sedimentation rate, gate

type, nature of the grated compartment and method of sludge extraction).6. Provide for easy access, maintenance and management of the facility from the outset of the design

phase.

This process appears essential to obtaining the best possible performance from the pollution control facilitiesrather than accepting merely average results: the study demonstrated that the Vigneux-sur-Seine facilitywould have performed more efficiently in Brunoy, and vice-versa. The automatic sludge extractionprototypes tested during this study require further improvements before they can be considered fully

operational .