Silt Laden Waters. Impacts and Treatment. Jonathan Goldsmith

Transcription

1 Silt Laden Waters Impacts and Treatment Jonathan Goldsmith

2 Things to Cover Why the concern?. 2. How big is the problem? 3. Environmental harm, how is it caused? 4. How clean is clean? 5. How can the problem be minimised? 6. How can we treat the problem? 7. Chemical treatment: coagulation and flocculation, how does it work?

3 Silt Pollution : Why Worry?... Silt pollution is highly visible. Easily traceable back to the site. Can travel a long way. Can cause significant environmental harm. Large quantities can escape. It s a common reason for construction company s going to court

4 Silt Pollution... Silt Pollution The world s most common and abundant pollutant.

5 Why the Concern? Erosion... The process by which the land surface is worn away by the action of water or wind. Sedimentation... The movement and settling out of suspended soil particles. Both processes have the potential to cause significant detrimental impact

6 Man Made vs Natural

7 How Common is Silt Pollution Silt Sewage % of Total Number of Incidents Sewage Average = 23 % SiltAverage = 27 % Year

8 Poor Management What could go wrong?

9 Pathway (Main River)

10 Receptor (Nature Reserve )

11 Pathway Down Stream

12 Pathway (Upstream)

13 And the source of the silty water was?

14 Highly Visible... Silt Pollution Highly visible and easily traceable back to site

15 Dalwhat Burn, Scotland... 0 km Escape Source: SEPA

16 Dalwhat Burn, Scotland... 1 km Down Stream Source: SEPA

17 Dalwhat Burn, Scotland... 4 km Down Stream Source: SEPA

18 The Problem... The Problem Silty Water Causes Silt is a broad term applied to fine particles of soil. Rainwater run off from topsoil stripped areas of site. Silt when mixed with water in the form of mud can be washed off construction sites into nearby drains and watercourses. Pumping out and dewatering of excavations. Cleaning of ditches and drains There are legal limits for the concentration of suspended solids which can be discharged. It is a criminal offence to discharge excessive concentrations of suspended particles. Works within river channels. The discharge of silt laden water is a major source of environmental pollution!!!!!

19 Silty Water The Harm... Environmental Harm Can suffocate fish by blocking gills. Can strip out dissolved oxygen. Prevent sunlight penetrating the water. Settle out on the bed of the river. Blinding off of gravel beds. Combine with other contaminants.

20 How Clean is Clean?... How Clean is Clean?

21 How Clean is Clean mg/l 500 mg/l 100 mg/l 60 mg/l 40 mg/l 10 mg/l

22 Preventing Pollution... Preventing Pollution What do we need to achieve?

23 Preventing pollution is simple... Source Three Key Aims Minimise the Source Pathway Eliminate/Reduce the Pathway Protect the Receptor Receptor

24 Preventing the Problem... Erosion Control Minimise Volumes of Dirty Water Keep Clean Water Clean

25 Minimising the Source... Minimising the Source

26 Minimising the Source... Minimise the time that soils are exposed. Reinstate/stabilise exposed surfaces as you go. Schedule and Phase the Works Schedule and phasing the works will limit the extent of soils exposed at any one time. Concentrate on an area. By intensively concentrating on an area you can speedily complete this area, before exposing the next. Limit the areas which may be exposed.

27 Siltfence... Silt Fence and Fibre Rolls

28 Silt Fence and Fibre Rolls... Silt fence and fibre rolls are temporary sediment barriers. Designed to retain sediment laden waters from small areas. Not designed to handle flows from large slopes or in areas of concentrated flows. Not suitable for installation within the stream channel. Do Use as perimeter controls, particularly at the lower or down slope edge of a disturbed area or around perimeter of stockpile area. Leave space for maintenance between the toe of the slope and barrier. Trench in the silt fence (uphill side) Install stakes on the downhill side. Curve the end barrier upgradient to help contain runoff. Don t Install in ditches, channels, or areas of concentrated flow Install it running up and down a slope or hill For large areas don t use as a stand alone solution. Use in connection with other treatment solutions.

29 Standard Silt Fence Installation

30 Silt Fence in Moving Water Hire, Sales & Technical Support

31 Ditches and Dikes... Ditches and Earth Banks

32 Diversion... Identify any Potential Surface water Flows Entering the Site. Divert these flows around the site. Divert runon and runoff away from disturbed areas. Ensure that the diversion is protected from erosion, using vegetation, geotextiles etc. Divert sediment laden water to an sediment trapping structure. Use practises that encourage infiltration of rainwater. If you are working in a mountainous area a significant proportion of the water leaving your site may be derived from up gradient areas.

33 Minimising Area (Reducing Water Volumes)... Annual Rain Fall: Soil Type: Return Period: 3,000 mm/year. Wet upland, shallow rocky soils on steep slopes, peat with impermeable layers at shallow depth. 1 in 10 year. Hectares 0.5 (Approx 70 m by 70 m) Peak Flow (m 3 /hr) , , (Approx 548 m by 548 m) 3,724

34 Earth Dikes/Ditches and Lined Ditches These are structures that intercept, divert and convey surface runoff, generally sheet flow, to prevent erosion. Applications Convey surface runoff down sloping land. Intercept and divert runoff to avoid sheet flow over sloped surfaces. Divert and direct runoff towards a stabilised watercourse, drainage pipe or channel. Intercept runoff from paved surfaces. Below steep grades where runoff begins to concentrate. Along roadways and facility improvements subject to flood drainage. At the top of slopes to divert runoff from adjacent slopes. At the bottom and mid slope to intercept sheet flow and convey concentrated flow. They may also be used to provide valuable storage capacity, buffering peak flow rates. Limitations Can not be used as sediment trapping devices. May require soil stabilisation and sediment controls, such as check dams, lining blankets, to prevent scour and erosion in newly graded/constructed situations.

35 Typical Site Practices...

36 Check Dams... Check Dams

37 Check Dams Check dams reduce scour and channel erosion by reducing flow velocity and encouraging sediment settlement. May be constructed out of: rock, gravel bags, sandbags, fibre rolls, straw bales, or other proprietary products. Applications In small open channels that drain less than 4 ha. In steep channels where storm water velocities exceed 1.5 m/s. During the establishment of grass linings in drainage ditches or channels. In temporary ditches where the short length of service does not warrant establishment of erosionresistant linings. Limitations Not appropriate for live streams. Not to be placed in channels that are already grass lined unless erosion is expected, installation may damage vegetation. Require extensive maintenance following high velocity flow. Promotes sediment trapping, which can be resuspended during subsequent storms or removal of the check dam.

38 Straw Bales

39 Check Dams

40 Infiltration... Infiltration

41 Discharge to Land (Infiltration).. Silty water spread to land (or sprayed onto land) Solids trapped in surface materials Water allowed to infiltrate to ground water Ground level Infiltration rate reduced by sediment clogging Cross surface flow Water Table Seepage to ground water (Controlled Waters)

42 Treating Water Locally...

43 Discharge to Land (Infiltration).. Range Typical values (mm/hr) (mm/hr) Sand Sandy loam Loam Clay loam Silty clay Clay 25 to to 75 8 to 20 3 to to 5 1 to Typical infiltration capacities l/s/ha m 3 /hr/ha

44 Infiltration... Area needed dependant on ground conditions (soil type). Doesn t work if your site is flooded/below the water table. Beware of land drains. Treat locally, number of small areas better than one large area.

45 General Rules of Use... Clean water that has gathered in the base of an excavation or clean water discharged from a settlement tank can be pumped out over adjacent ground as long as: It is only pumped over land that is part of your site or you have permission from the land owner to pump onto. Pump at a rate that allows the water to be quickly absorbed into the ground. Move the discharge point around to prevent ponding. Don t pump onto land that is saturated, this can lead to pollution if water flows overland and into a water course. Do not pump onto areas that have been stripped of vegetation or topsoil as the water is likely to pick up high levels of silt. If this water enters a watercourse it could cause pollution.

46 InStream Mats Traps harmful disturbed sediments (Average 80%) Protects aquatic habitats Does not cause water backup Easy to handle, install and remove Completely biodegradable after use Provides useful bank stabilisation and protection Install the 1.20 x 3.00m mats across the full width of the stream Faster moving water requires a greater length of coverage than slower moving water (especially turbulent flow) Silt/clay beds require a greater length of coverage than if sediments are primarily sand/gravel The number of mats is directly proportional to how invasive the project will be Water Velocity 00.3m/s m/s m/s >0.9m/s Fines mostly sand 1.20m 2.40m 3.60m >5.00m Fines mostly silt and clay 2.40m 4.80m 7.20m >10.00m

47 InStream Mats

48 Active Treatment... Active Treatment The last line of defence

49 Active Treatment... Despite all of our best efforts due to prevailing weather conditions we will have a need to discharge water. Prior to discharge, treatment to ensure that the water achieves the required water quality discharge criteria is needed. Dirty water should not be allowed to escape into the environment.

50 Filtering of Silt Laden Waters Filtration Techniques

51 Geotextile/Media/Cartridge Filters Rely on passing the water through a fine media to trap the solids. Geotextile Paper cartridges Fine metal mesh Hydraulic capacity rapidly decreases as layer of solids build up on the surface of the media (minutes/seconds at high solids loads). Need frequent replacement hence not used on construction sites.

52 Dirtbag/DirtBox.. Pore sizes 100, 200 or 500 micron. Low solids storage capacity. Handling difficulties. Reduced pump performance. Only suitable for removal of low concentrations of coarse solids.

53 Filtering of Clay Particles

54 Treated Discharge Water

55 Receiving Watercourse Hire, Sales & Technical Support

56 Hydrocyclones Hydrocyclones

57 Hydrocyclones

58 Settlement Lagoons and Settlement Tanks... Settlement Lagoons and Tanks

59 Typical Settlement Solutions.. Not Pretty but Why?

60 Silt Settlement Theory. Solids removal is Area related not Volume related Q = V x A Where. Q = Treatable flow rate (m 3 /hr) V = Settling velocity of particles (m/hr) A = Effective settling area (m 2 ) Need to maximise: SETTLEMENT AREA PARTICLE SETTLING VELOCITY

61 Silt Settlement Theory... Solids Removal is Area not Volume Related Two tanks, same volume different shape. For a given Flow and Particle Size: Larger Area particle settles Smaller Area same particle is swept over

62 Settlement Area vs Effective Settlement Area...

63 Settlement Tanks. Small and portable Often purpose made with baffles to still flow and maximise flow path Baffles also trap oil Limited effectiveness due to Small surface area Solids separation zone Inlet baffle Outlet baffle

64 Portable Lamella Clarifiers.. Compact Proven Technology Large Settlement Area Small Footprint Small Settlement Distance No Wind Induced Turbulence Floating Material Retention

65 Comparison lamella clarifier with conventional settlement tank 1 Siltbuster FB50 (1.5mx3.5m) 10 to 20 Conventional Tanks (2m x 3m)

66 Settlement Tanks in Series.. Q Tank A Tank B Q If area of Tank A = Area of Tank B Little additional solid removal occurs in B

67 Settlement Tanks in Parallel.. Q/2 Tank A Q Q/2 Tank B Q Hence better clarification if Operated in Parallel

68 Smaller particles lower settlement velocity... Material: SAND SILT CLAY Typical diameter: (2mm) (63μm) (2μm) m/hr 11 m/hr Settlement area proportional to settling velocity 1000 m/hr Sand to clay variation of 100,000(!)

69 Implications of Particle Size... Diameter Settlement Area Area for 50m 3/ hr Gravel 2mm 0.02m 2 /m 3 1m 2 Sand 63μm 0.1m 2 /m 3 5m 2 Medium silt Fine silt (cement) 20μm 1m 2 /m 3 50m 2 10μm 5m 2 /m 3 250m 2 Clay 1μm 100m 2 /m 3 5,000m 2 (Based on a particle density of 2.65t/m 3) Conclusion: Maximise available area and/or increase settlement rate

70 Effect of Particle Size on Settlement Area SAND... Settlement area required to remove SAND particles at 50m 3 /hr

71 Effect of Particle Size on Settlement Area CLAY. Settlement area required to remove CLAY particles at 50m 3 /hr

72 Selecting Treatment Systems.. Undertake Settlement Test in 1 litre bottle Solids Settle in less than 1 Min Solids Settle in less than 10 Min Solids Settle in less than 20 Min Solids Settle in more than 20 Min Conventional Settlement Tank Poorly Settling Solids. Hydrocyclone Siltbuster Unit Settlement Lagoon Hydrocyclone Siltbuster Unit Settlement Lagoon Siltbuster Unit Settlement Lagoon Assess need for chemical pretreatment to modify/improve settling characteristics Coarse Fine

73 Poorly Settling Solids Chemical PreTreatment Modifying Settling Velocities

74 Treatment of Nonsettling Solids... Interparticle repulsion forces can be overcome by chemically pretreating the water using: Coagulant Flocculant Combination of the two Electro coagulation This aggregates the particles into larger more rapidly settling units These can then be removed by conventional gravity separation techniques

75 Coagulation... Fine clay particles have electricaly charged surface layers Particle unable to aggregate due to electrical repulsion Addition of a coagulant destabilises electrical charge field allowing particles to aggregate + +

76 Coagulation... Typical coagulants are positively charged metal ions: Ferric iron (Fe 3+ ) Ferrous iron (Fe 2+ ) Aluminium Sulphate (Al 3+ ) Poly Aluminium chloride (PAC) + +

77 Flocculation... Addition of long string organic compound Bind particles together Forms larger more rapidly settling aggregated units Charged sites bound to particles Long string molecule

78 Flocculation... Typically 4 types Anionic Negatively charges sites Used on mineral/coagulated material Low toxicity to aquatic life Cationic Most commonly used on organic material High toxicity to fish Non ionic Natural flocculants Derived from tropical tree bark and prawn shells Tends to be expensive

79 Practical Demonstration. Options for Adding Water Treatment Chemicals

80 Options for adding chemicals... Treatment Chemicals can be added in one of three forms: Floc Blocks Electro Coagulation Liquid Dosing

81 Options for adding chemicals...floc Blocks A solid block of treatment chemicals dispersed within a solid matrix. On contact with water the matrix dissolve at a constant rate releasing the treatment chemicals into the water where they react with suspended solids. Coagulant Need to be suspended within water column to prevent block becoming blinded off with sediment. Only suitable for applications with constant flow rate and constant sediment loadings. Polymer May need to be deployed in conjunction with a flow balancing structure to ensure that flow rate passing over the block is constant. Low tech solution, with little control over dosing rates. Coagulant and Polymer

82 Electro coagulation... (1) Particle surface charge neutralised by a small electrical current Current typically dissolves iron or aluminium plates Dissolved metal precipitated as hydroxide Acts as a coagulant Power supply Associated electrochemical reactions Raise/regulate ph Destroy COD by electrochemical oxidation Break down other complex compounds Fe 3+ 2H + +2e = H 2 Fe = Fe 3+ +3e

83 Liquid Dosing System... The addition of treatment chemicals in liquid form. Precision control over the chemical dose added. Remote monitoring of system performance possible. Secure storage of treatment chemicals and dosing equipment. Dose rate can be varied in response to changes in flow rate and suspended solids content. Ideally suited to a construction environment

84 What Do You Need from a Chemical Treatment System... Site Conditions Only small amounts of chemicals are needed to adjust the settling characteristics of suspended solids (typically 1 to 10 mg/l (mg/l = part per million). The use of excessive chemicals is not advantageous and can cause treated water quality to decrease. The flow passing through the treatment system will be highly variable (Zero m 3 /hr to peak flow conditions). Significant variations in flow rate can occur over a short period of time. Treatment chemicals need to be added in proportion to the flow rate. During low flow rate conditions the addition of treatment chemicals may not be required.

85 Constant Rate Dosing... Constant Dose Rate Constant Dose Rate 50 g/minute

86 Flow Proportional Dosing... Flow Proportional Dosing 50 mg/l

87 Practical Demonstration. Seeing is Believing

88 Any Questions Jonathan Goldsmith