GUIDANCE FOR DESIGN AND CONSTRUCTION o; A SUBSURFACE FLOW CONSTRUCTED WETLAND U.S. EPA-REGION 6 WATER MANAGEMENT DIVISION MUNICIPAL FACILITIES BRANCH TECHNICAL SECTION AUGUST 1993 ------- Addendum 1 to Document, Guidance for Design and Construction of a Subsurface Flow Constructed Wetland In the initial publication of this document, the recommended size of the wetland media was 2-5 inches (50.8-127 mm) in diameter. Research conducted since the initial publication has shown that smaller media, gravel size, 0.5-1 inch (12.7-25.4 mm) supports better growth of the plant root systems than does the larger media originally recommended. As a result, the recommendation now is to use smaller gravel as the media. Since the hydraulic conductivity is dependent on both the rock and void space size, use of a smaller media will decrease the hydraulic capacity of a system in comparison to that of the larger media. To offset the loss of hydraulic capacity resulting frou the use of the smaller media, change the surface configuration of the wetland to decrease the length and increase the width. Using Darcy's equation, determine the flow that can pa?- through pach configuration change (length and width dimensions) of the wetland and compare this with the design flow. If the flow does not equal or exceed the design flow, adjust the length and width and repeat the process. The optimal point would be where the wetland flow equals the design flow. System flows exceeding the design flow could result in systems being larger than necessary. Typical media characteristics for fine gravel are as follows: 1. Effective Size - 0.67 inches (16mm) 2. Hydraulic Conductivity - 24,608 ft3/ft2/day (7,500 m3/m2/day) 3. Use the porosity value - n suggested in Chapter III, Page 10. Use of smaller media will ^.Iso affect the recommended gradation found 01. Page 17. Based on the new inedii size recommendation, use the following gradation: Recommended Gradation for SFCW Media 40% 0% - 80% 100% Retained Retained Retained 1.0 3/4 1/2 inches inches inches [25 [19 [12 .0 .0 .b mm] mm] mm] sieve sieve sieve | For surface layer gradation, use the following: Recommended size of surface media 0.5 to 0.75 inches [12.5 mm 19.0 mm]. Gradation of this size is not as critical as for the media, however, care should be taken not to have media size less than 1/2 inch. Smaller media can migrate through the filter media an cause a loss of void space. It is also critical to make sure that the stone is washed to remove all fines. ------- TABLE OP CONTENTS Foreword i I Subsurface Flow Constructed Wetlands Technology 1 Overview 1 SFCW 1 Types of Precedent Treatment 3 II Factors Capable of Influencing Performance Expectations . 5 III Design and Construction Considerations for SFCW 9 Calculating Design Requirements 9 Reoommended Design and Construction Considerations 16 IV Operational Considerations 20 ------- FOREWORD This manual is intended to provide guidance in the selection, design, construction, and operation of subsurface flow constructed wetlands. The guidance information contained herein is considered necessary due to the proliferation of the use of this technology, particularly by small communities, where there has been considerable effort to construct and maintain treatment facilities cost-efficiently, capitalizing on the minimal energy requirements of this technology. The technology is also considered an attractive alternative for use.in unsewered areas such as the colonia developments along the U.S.-Mexico border. In addition, it may also have applications in other similar situations internationally. It is recognized that problems have developed with the use of this technology in some installations. This manual will discuss probable causes for the problems ^nd will delineate guidance in an attempt to avoid the problems that have been experienced. The guidance material contained in this document is the result of the operating experience and observation of existing subsurface flow constructed wetlands in the Region 6 area and the work performed under contract to the Environmental Protection Agency by Mr. Sherwood C. Reed, P.E. The contribution of Mr. Ancil A. Jones' recognition of the value in the application of this technology is gratefully acknowledged. Mr. Jones dedicated himself to the use of new and innovative technologies for their cost and energy saving characteristics, especially in small community applications. Mr. Jones recognized the potential for use of the subsurface flow wetlands technology when he was introduced to experimental work conducted by the National Aeronautics and Space Administration for possible use in space stations. This manual is respectfully dedicated to the memory of Mr. Jones who passed away during the preparation of this manual in March 1993. For additional information, you may contact: Chief, Municipal Facilities Branch (6W-M) Water Management Division U.S. Environmental Protection Agency, Region 6 1445 Ross Avenue Dallas, Texas 75202-2733 ------- CHAPTER I SUBSURFACE FLOW CONSTRUCTED WETLANDS TECHNOLOGY OVERVIEW The casual observer may ask, "What is a subsurface flow constructed wetland?" The answer to this question is as follows: A subsurface flow constructed wetland (SFCW), as discussed in this manual, consists of a bed of graded stone media with an influent distribution device to introduce wastewater at one end of the bed of stone and an effluent collection device at the opposite end of the bed to collect and discharge the treated effluent from the bed. Vegetation in the form of plants strategically placed on the surface of the bed of stone may also be used. Microbes attach themselves tc the stone media, and, if plants are used, to the root systems of the plants. These microbes are useful in synthesizing dissolved organics in the wastewater thereby providing treatment. The dissolved organics are synthesized by the microbes to provide cellular growth. Some oxygen is believed to be provided by the plants through the root systems but the actual amounts provided are not known at this time. The SFCW technology as used in wastewater treatment system applications is generally credited to the experimental work accomplished by the National Aeronautics and Space Administration in efforts to recycle wastewater in space stations. The use of SFCW technology by small communities and other relatively small entities seeking a cost-effective, energy efficient, and relatively unsophisticated method of wastewater treatment has resulted in a proliferation of the use of the systems. Many of these systems have experienced problems in consistent performance with the passage of time. This manual is intended to identify possible causes of the problems encountered and to recommend design, const.ruction, and operations characteristics in an attempt to avoid the problem areas. If no plants are used in a SFCW, it is commonly referred to in Region 6 as a microbial rock filter (MRF). If vegetation in the form of plants are used in a SFCW, it is referred to as a microbial rock plant filter (MRPF). SFCW CONFIGURATION The SFCW with the use of plants is schematically represented in Figures 1 and 2. ^riginally, it was believed that a large length-to-width ratio on the order of 10 to 1 or greater was desirable to prevent the possibility of short circuiting of the ------- TYPICAL SUBSURFACE FLOW CONSTRUCTED WETLAND PLAN VIEW INFLUENT \ EFFLUENT — . o o 0 0 . 0 0 FLOW o 0 -T O O 10 0 0 o.cl — -*-O o .10 OC; o o o o o 0 o o o 0 0 0 o o o o 0 NOTE PLANT DENSITY SHOULD BE MAINTAINED AS SHOWN ADDITIONAL PLANTS THAT MIGHT DEVELOP AND REDUCE THE ORIGINAL PLANT SPACING SHOULD BE REMOVED FIGURE I PLANT FREE ZONE 10' LONGITUDINAL SECTION y N/ 6' LAYER OF •3/4'-\ 1/2' ROCK PLANT FREE ZONE 10' 880. jg^gQP pffA3pQ L J I J-S-'^.y^L oo DO. PERFORATED INLET PIPE 2' - 5' ROCK HOLD DOWN STRAP PERFORATED OUTLET ANCHORS P1PE VOLUME • MINIMUM OF 24-HR DETENTION VOID SPACE WITHIN 2'-5' ROCK IS 35% WHEN PLANTS ARE USED AND 45% WHEN NO PLANTS ARE USED IT IS SUGGESTED THAT PLANTS BE PLACED ON 10 FT. CENTERS FIGURE 2 ------- wastewater through the filter. Experience has revealed that a long length-to-width ratio can adversely impact the hydraulic gradient of the stone bed. Today, lower length-to-width ratios are preferred, particularly in the larger systems. Short circuiting can be prevented by segmenting the SFCW into smaller parallel components. The depth of the stone bed has characteristically been 24 to 30 inches. There is some evidence to indicate that the bed depth can be 18 inches and possibly avo^d anaerobic conditions at or near the bottom of the bed. The influent device is typically a perforated or slotted plastic pipe extending across the width of the bed in order to distribute the flow across the entire width of the system. It is recommended that the pipe be located at or near the top of the stone bed. The design of the influent pipe is very important due to the need for the flow to be e\enly distributed across the width of the bed. f The effluent collection device is usually a perforated or slotted plastic pipe extending across the width of the bed to uniformly collect the effluent. It is recommended that the pipe be located at or near the bottom of the bed of stone. The outlet pipe at the effluent end of the SFCW is recommended to be capable of elevation adjustment to enable raising or lowering the water level in the bed of stone. Types of plants that have been used in SFCW systems in Region 6 include the following: • Southern Bulrush (scirpus californicus) • Reed (phragmites communis, • Pickerel Weed (pontederia cordata) • Arrowhead (sagitaris spp.) • Soft Rush (juncus effusus) • Water Iris (iris pseudacorus) • Duck Potato (sagittaria falcata) • Canna Lily (canna flaccida) • Calla Lily (zantedeschia aethiopical) • Thaylia (dealbata and divericata) There is no data available on the amount of oxygen that may be released in the root zone of the plants. There is reason to believe that some oxygen is released and is an aid in the maintenance of an aerobic condition in the wastewater in the bed of stone. ------- TYPES OF TREATMENT THAT MAY PRECEDE THE SFCW Lagoon Precedent Treatment In a lagoon system preceding the SFCV7, the lagoon treatment works should be designed and constructed to achieve the maximum degree of treatment feasible consistent with acceptable criteria for lagoon systems. Effluent works are recommended to be arranged to minimize the discharge of solid materials from the lagoons to the SFCW. It is recommended that applicable sources of design information for lagoon systems be consulted with appropriate state agencies. The EPA Design Manual, "Municipal Wastewater Stabilization Ponds", EPA-62511-83-015 is recommended as a reliable source to use. Septic Tank Precedent Treatment Septic tanks may be used as a precedent treatment process. The tanks should be sized appropriately to maximize the reduction of settleable solid materials and should be serviced at proper intervals to maintain the removal efficiency. The appropriate state agency should be consulted regarding the design and construction of septic tanks. In general, the concentration of the organics in the effluents from the septic tanks will be much greater than the concentration of organics in the effluent from the lagoons. The higher concentrations in the septic tank effluents will necessitate longer detention times in the SFCW's to achieve the desired effluent qualities. ------- CHAPTER II FACTORS CAPABLE OP INFLUENCING PERFORMANCE EXPECTATIONS Based on data gathered from existing facilities as shown in Table 1, the performance of the SFCW, Microbial Rock Plant Filter (MRPF), appears to be quite attractive. However, it must be realized that much of the data in Table 1 was obtained within one to two years after initiation of operation of the facilities. Since the data was obtained, the performances of some of these facilities have deteriorated. It is important to note that many of these earlier systems were possibly designed by means of empirical information and without reference to rational design procedures and were constructed with inadequate hydraulic gradients, materials that did not provide sufficient hydraulic conductivity, and construction practices that later proved detrimental to long term performance efficiency. These potential problem areas were not recognized at- the time of conception or construction and have only become apparent as the experience base has increased with time as may be expected with any relatively new technology. The causes of some of the impaired performances are believed to be inadequate control over the gradation and sizing of the stone media, the improper type of stone media, the failure to remove fine particles from the stone media by washing prior to construction, improper construction practices such as running heavy equipment repeatedly over the stone media after it was in place, failure to line the media bed with an impervious liner, failure to protect against erosion of side slopes resulting in fine material entering the stone media and contributing to clogging, inadequate hydrauMc gradient to accommodate the volume of flow, improper placement of thp. influent and effluent works, improper spacing of plants which may have contributed to clogging of the stone media, failure to remove plant detritus from the stone media, and improper sizing of the SFCW to allow for loss of media void space with time. It is believed the deterioration of the performance of some of the systems, with the passage of time, has resulted from a combination of the above mentioned causes. It should be noted that some of the facilities exhibiting performance deterioration are still meeting their effluent requirements. "Pertinent design features of the facilities contained in Table 1 are shown in Table 2. It is significant that a very flat hydraulic gradient will exist for those facilities with a filter bed depth of 2 feet or less and a large length-to-width ratio. Systems with large length-to-width ratios do not provide sufficient hydraulic gradient to force the wastewater flow through the stone ^edia and plant roots, and many have experienced impaired performance over time. ------- With proper attention to the hydraulic design features and the organic removal capabilities, the systems should be able to provide biochemical oxygen demand (BOD) removals in the range of 85 to 90%. This translates to anticipated effluent qualities as low as 5 mg/1 BOD dependent upon the BOD of the influent to the SFCW. With the proper hydraulic design and organic removal capabilities, proper media selection, sizing and gradation, proper washing of the media, adequate preparation and lining of the filter bed, protection against side slope erosion, careful placing of the media during construction, proper arrangement of the influent and effluent collection devices, and adequate plant spacing should result in a system capable of long term performance with very little deterioration in performance with the passage of time. It is also important to recognize that certain management practices are necessar1 to insure continued long term performance. Practices such as removal of plant detritus from the filter media surface and periodic thinning of plant growth may be necessary. Removal of undesired extraneous vegetative growth may also be required periodically. Trimming some types of plants may be in order to encourage continued plant growth. If elevation adjustment capability is included in the effluent collection device, periodic adjustment may assist the system performance. The capability to reeirculate the effluent back to the preceding treatment or to the influent end of the SFCW is a recommended feature. Adequate attention to the above factors influencing the performance of SFCW's should result in a system with long term and efficient treatment capabilities. ------- TABLE 1 MICROBIAL ROCK PLANT FILTER - SYSTEMS DATA (Extracted from Draft Report Dated February 1993, Prepared by Sherwood C. Reed, P.E.) MUNICIPALITY Greenleaves Subdivision (Mandeville LA) DeGussa Corporation (Mobile AL) Phillips High School (Bear Creek AL) Monterey V? Denham Springs LA B^nton LA Haughton LA Carville LA Mandeville LA Benton KY Hardin KY (Phragmites Side) Hardin KY (Scirpus Side) Utica MS (North) Utica MS (South) INFLU TOTAL BOD, •g/1 36 5 13 39 25 10 12.5 20 41 26 51 51 . 38 31 EFFLU TOTAL BOD, •g/1 12 4 1 15 10 6 2 8 10 9 9 4.1 14 11 INFLU TSS mg/1 42 23 60 32 48 57 47 93 59 56 118 118 52 32 EFFLU TSS •g/1 10 4 3 7 14 4 14 17 7 4 17 9.4 23 11 INFLU NHj~ll •9/1 4.2 10.0 9.3 0.7 0.6 1.1 4.8 1.4 5.1 10.1 10.1 6.7 5.6. EFFLU NH,-N mg/1 2.3 2.0 8.0 10.0 2.8 7.2 5.1 2.1 7.4 9.9 8.3 2.9 3.1 INFLTT NO, •g/1 26.0 0 4.4 14.4 0.5 0.5 0.3 0.3 EFFLU NO, •g/1 6.0 0 0.8 9.8 0.3 0.3 0.2 0.2 FECAL COLI #/ 100 .1 10 3800 700 ------- TABLE 2 MICROBIAL ROCK PLANT FILTER - SYSTEMS DATA (Extracted from Draft Report Dated February 1993, Prepared by Sherwood C. Reed, P.E.) MUNICIPALITY Creenleavea Subdivision (Mandeville LA) DeGuaea Corp (Mobile .AL) Phillip* High School (Bear Creek AL) Monterey VA Denham Sprnga LA Bent on LA Haughton LA Carville LA Mandeville LA Benton KY Hardin KY (Phragmitea Side) Hardin KY (Scirpua Side) Utica MS (North) Utica MS (South) t PLOW NOD 0.149 1.78 0.0155 0.022 1.73 0.1 0.1 0.1228 1.224 0.1881 0.062 0.0492 0.05 0.11 FILTER SURFACE AREA ACRES 1.1 2.2 0.502 0.056 15.2 1.2 1.5 0.64 4.56 3.6 0.79 0.79 1.5 2.0 FILTUt union FEET 457 475 175 74 1050 90C 934 528 470 1092 475 475 280 315 FILTER WIDTH FEET 105 28 125 33 630 58 72 G2 207 144 72 72 140 158 LiM RAXIO 4.4:1 17.0:1 1.4:0 2.2:1 5.0:1 15.5:1 13.0:1 8.5:1 2.0:1 7.6:1 6.6:1 6.6:1 2.0:1 2.0:1 FILTER DEPTH FEET 2 2 1 3 2 2 2.5 2.5 2 2 2 2 2.1 2.1 FILTER CROSS SECTION AREA FT1 210 56 125 99 1260 lib 180 155 414 288 144 144 294 332 FILTER CROSS SECTIONAL BOO LOAD LB/DAY/PT* 0.213 1.3 1.68 0.07 0.29 0.13 0.058 0.13 1.01 0.14 0.18 0.15 0.05 0.09 FILTER SURFACE AREA-RYD LOADING OAL/DAY/FT1 3.1 133.8 0.71 9.0 2.62 1.9 1.49 3.75 12.6 1.15 1.81 1.44 1.28 2.21 PRELIM TREAT Acnud Lifooi Oxid* Dfeb Brt.^ AM* Imboff Tuk FKU| LMOO. Fieri L*fooa F«ul Ufoo. A*nud Lafooa 3C*II Aenud F*c«l UfOOfl Coaucl Sub CoMMt Sub Facal Ijigoem Ptat Ufoo. DESIOM BRT DAYS 1.0 1.0 3.9 0.9 1.0 2.1 4.5 1.4 0.7 5.0 3.3 4.2 5.0 3.7 oo ------- CHAPTER III DESIGN AND CONSTRUCTION CONSIDERATIONS FOR SUBSURFACE FLOW CONSTRUCTED WETLANDS The information in this chapter pertains to suggested procedures for the design and construction of a SFCW for the reduction of biochemical oxygen demand (BOD) and total suspended solids (TSS). While there are no specific design equations relating solely to TSS removal in this information, historically the TSS concentration in the effluent from SFCW's has shown a close correspondence to the concentration of BOD in the effluent. This chapter does not include suggestions for design procedures to remove ammonia, achieve denitrification, phosphorus reduction, or reduction of fecal coliform. Additional research may be necessary to arrive at criteria for usa in design procedures for th-jse items. Hydraulic characteristics play a vital role in the successful performance of a SFCW system. The importance of uniform distribution of the influent flow across the cross section of the bed of stone cannot be overemphasized. The hydraulic gradient through the system must be sufficient to drive the flow through the media in a subsurface mode. The gradation of the media must provide sufficient void spaces to accommodate the flow quantity. Therefore, adequate provisions must be made to allow for some loss of void spaces due to root growth if plants are used and for some solids accumulation. CALCULATING DESIGN REQUIREMENTS The process of designing a SFCW involves several steps which are discussed in the following paragraphs. Final sizing of the filter may require several iterations of this process. Suggestion of specific criteria are based on observed results in existing facilities in Region 6. 1. Determine the existing conditions (influent BOD, TSS, average winter temperature, average daily influent flow. 2. Determine the desired quality of the effluent (BOD and TSS) . 3. Select bed depth (sugcrest a maximum of 2 feet [0.62 m) of filter media), media type, and size (use a hard, insoluble rock, 2-5 inches in diameter). ------- 4. Select a value for the void space within the rock media. n = 0.35 if plants are used n = 0.45 if no plants are used 5. An initial length-to-width ratio of the SFCW must be selected based on the area calculated to achieve the desired BOD reduction. It is suggested that a length-to- width ratio of 2:1 be selected initially. The final overall length-to-width ratio depends on the hydraulic considerations. 6. Calculate the surface area required using the first order BOD removal equation given as Equation 1. As = (L) (W) = c;in(C0/Ce) ] :- Ktdn (Equation I)1 -2°° C Kt - K2Q= 1-104 0 = 1.06 "Where: As = Surface Area of SFCW (Ft2)[m2] L = Length (Ft)[m] W = Width (Ft)[m] Q = Flow (Ft /day)[m3/day] Co = Influent BOD (mg/1) Ce = Effluent BOD (mg/1) Kt = Rate constant at wastewater temperature T°C K20= Rate constant at wastewater temperature T = 20°C d = Average depth of water in filter (Ft)[m] n = Porosity of filter media (% as a decimal) 7. After determining the surface area required and the corresponding dimensions based on the initial length-to- width ratio, use Darcy's Equation to determine the capability of the design to conduct the flow through the SFCW. 1 Reed, Sherwood C., P.E., "Subsurface Flow Constructed Wetlands for Wastewater Treatment," Draft, February 1993 10 ------- Q = KSAS(Equation 2)* Where: Q = Flow capable of being passed through SFCW (FT3/day) [m3/day] Kg = Hydraulic conductivity of a unit area of the media (for 2-5 inch [50.8-127 mm] media Ks = 328,100 Ft3/Ft2/day [100,000 m3/m2/day]) (suggest using 0.3 Ks as a safety factor) S = Hydraulic gradient of the water surface in the system (d/L) (suggest using 0.1 of maximum S as a safety factor) A = Cross-sectional area of SFCW (Ft2) [m2] If Q in Equation 2 does not equal or exceed the design flow, the length-to-width ratio must be ad_ usted to decrease the length while increasing the width to maintain the surface area determined by Equation 1. This process is repeated until the design flow is less than or equal to the flow determined by Equation 2. NOTE: Anytime the length-to-width ratio is adjusted, the hydraulic flow capability should be checked by Darcy's Equation (Equation 2) . „________________„ In. some cases, the larger facilities will require a width larger than the length. In these cases, it is suggested that the filter be partitioned into several smaller filter units so that each individual filter would have tlie length greater than tho width. This would aid in preventing short circuiting within the filter. The following are examples using the above design procedures: Example 1 - Construction of New System with Lagoon Precedent Treatment A small unsewered community must build a sewer system to treat their wastewater. The State Water Pollution Control Agency requires them to discharge wastewater containing no more than 20 mg/1 BOD and 20 mg/1 TSS. The estimated flow to the treatment facility is 50,000 gal/day [189.3 m /day] and the average wintertime temperature is 4.5° C. Further assume that precedent treatment is by a 2-cell lagoon that is designed to produce an effluent of 30 mg/1 BOD and 90 mg/1 TSS (required by the State). What size SFCW would be needed? 2 Ibid. 11 ------- 1. Existing conditions a. Influent quality BOD - 30 mg/1 TSS - 90 mg/1 Flow - 50,000 gal/day = 6,684 FtJ/day [189.3 nr/day] b. Effluent quality BOD - 20 mg/1 TSS - 20 mg/1 2. a. Bed depth - 2 ft [0.61 H] b. Media size range - 2-5 inches [50.8-127 mm] diameter granite c. Media porosity - 35% (with plants) d. Initial length-to-width <:atio - 2:1 3. Calculate the surface area required for BOD removal (Equation 1) As = (L) (W) = Q[ln(C0/Ce)] -5- Ktdn(Equation 1) Where: Kt = K2o (0)T~2°°C T = 4.5°C 0 = 1.06 K2Q= 1,104 Kt = 1. 104(1. 06)4*5°~20° Kt = 0.447 d = average water depth in filter d = 1 Ft [0.305 m] n = 0.35 Based on these values, the length and width calculations are as follows: L x W = (6,684 Ft3/day)[ln (30/20)] -s- (0.447)(1 Ft) (0.35) L x W = 17,322 Ft2 12 ------- L = 2W 2W.X W .= 17,322 Ft2 W = (17,322 Ft2 -s- 2)°'5 W = 93 Ft [28.3 m] L = 2W L = 2(93 Ft) L = 186 Ft [56.7 m] 4. Calculate the hydraulic flow using Darcy's Equation Q = KS AS (Equation 2) Where: Ks = 328,100 Ft3/Ft2/day [100,000 m3/m2/day] (Use 0.3 KS as a safety factor) Ks = 109,366 Ft3/Ft2/day [33,335 m3/m2/day] S = 2 Ft -r 186 Ft = 0.011 (Use 0.1 S for safety factor) S = 0.0011 A = 2 Ft X 93 Ft = 186 Ft2 Q - (109,366 Ft3/Ft2/day)(186 Ft2)(.0011) Q = 22,376 Ft3/day [S33.6 nr/day] Notice, in this example, the hydraulic capacity of the designed filter is 22,376 ft /day [633.6 m /day] whereas the average daily flow is 6,684 ft /day [189.3 m3/day]. Where a disparity this large exists between design and need, the designer should revise the cross-sectional area of the filter to approach optimal design. Using equation 2, with a depth of 1.5 ft, calculate the hydraulic capacity: Q = KSAS Ks = 328,100 Ft3/Ft2/day x 0.3 Ks = 109,366 Ft3/Ft2/day S = 1.5 Ft -H 186 Ft 13 ------- S = 0.008 0.1 S = 0.0008 A = 1.5 Ft X 93 Ft A = 140 Ft2 Q = (109,366 Ft3/Ft2/day X 140 Ft2) X (0.0008) , ' Q = 12,249 Ft3/day [347 m3/day] • The hydraulic capacity of 12,249 is still much larger than the needed 6,684. Continuing this revision process will yield the optimal design configuration of 186 ft long, 93 ft wide, and 1/25 ft deep with a hydraulic capacity of 8,881 ft /day. It is suggested that '-'here possible, the SFCW be partitioned into multiple cells, separated by berns wide enough, to accommodate heavy equipment. This would allow for placement of the stone media and maintenance of the SFCW without having heavy equipment on the media itself. Such partitioning will also promote uniform flow within each cell to facilitate treatment and to allow at least one cell to be taken out of operation for maintenance. Using this suggestion with the above example, the SFCW could be divided into three cells, each being 31 ft x 186 ft, with a stone media depth of 1.25 ft. In summary, it should be noted that each design will need to follow the process outlined in the above example. It will take several steps to determine the optimal sizing for a facility but careful consideration of these design suggestions and procedures should minimize the problems that have occurred at existing facilities. Detention Time Calculation of the theoretical detention time is as follows: Detention time = (Volume x void space) -*• (Flow) = {(186 Ft)(93 Ft)(2 Ft) x 0.35} + 6,684 Ft3/day Detention time = 1.81 days 14 ------- Example 2 - Design a SFCW to Receive Septic Tank Effluent A SFCW is proposed to be added to a septic tank. This on-site system would be sized for a typical family of four and an effluent flow per capita of 80 GPD [0.303 m /day]. The quality of the effluent desired from the SFCW is 20 mg/1 BOD and 20 mg/1 TSS. The average wintertime wastewater temperature is 4.5°C. 1. Existing Conditions Organic loading/capita =0.17 Ibs BOD/cap/day [77.1 grams BOD/cap/day] Loading = 0.17 Ibs/cap/day x 4.0 persons/residence = 0.68 Ibs BOD/day [308.4 grams BOD/day] Flow = 80 gal/cap/day x 4.0 persons/residence = 320 gal/day or Flow = 43.0 Ft3/day [1.2k1 i /day] Concentration of BOD to septic tank = 0.68 Ib BOD/day + 0.00032 MGD x 8.34 Ib/gal x 1 ppm/mg/1 Concentration of BOD to septic tank = 255 mg/1 Removal of BOD in septic tank = 30% (Assumption) Concentration of BOD to filter = 0.70 x 255 = 180 mg/1 2. Calculate the surface area required for BOD removal. AS = L x W = Q[ln(C0/Ce)] - Ktdn (Equation 1) Where: a.T-20-C Kt = K20 ( K2Q= 1.104 <=) = 1.06 T = 4.5°C Kt = (1.104)(1.06)4'5"20 = 0.447 d = 1 Ft [0.305 m] n = 0.35 L X W = (43 Ft3)[ln(180/20)] {(0.447)(1 Ft)(.35)} L X W = 604 Ft2 [56.2 m2 ] 15 ------- Assuming a 10:1 length-to-width ratio L = 10W 10W x W = 600 Ft2 W = (600 H- 10)°'5 W = 7.8 Ft [2.38 m] Length = 78 Ft [23.8 m) 3. Calculate the hydraulic capacity with Darcy's Equation. Q = Ks AS (Equation 2) Where: A = 2 Ft x 7.8 Ft = 15.6 Ft2 [1.45 m2] Ks = 328,100 Ft3/Ft2/day (Ura 0.3 Ks as a safety factor) Ks = 1/3(328,100) = 109,366 Ft3/Ft2/day [33,335 m3/m2/day] S = d -s- L = 2 Ft -r 78 Ft S = 0.026 (Use 0.1 as a safety factor) S = 0.10(0.026) = 0.0026 Q= (109,366)(15.6)(.0026) Q = 4,436 Ft3/day [125.6 m3/day] Based on the above calculations, the design should obtain the necessary BOD removal and provide adequate hydraulic capacity. By using the same procedure as in Example 1, the depth of the on-site SFCW can be adjusted to optimize the volume of the SFCW while maintaining the BOD removal capability. Detention Time Calculation of the theoretical detention time is as follows: Detention time = (Volume x void space) + (flow) = (7.8 Ft)(78 Ft)(2 Ft) X 0.35 * 43 Ft3/Day Detention time =9.9 days 16 ------- RECOMMENDED DESIGN AND CONSTRUCTION CONSIDERATIONS In developing the SFCW, several design and construction featuresshould be considered to enhance the ability of the constructed facility to function properly and to improve the operational capability. Gradation of Media The SFCW has two layers of graded stone media: (1) a subsurface layer to which the wastewater flow should be confined and (2) a surface layer which is used as a base to hold the plants. The subsurface layer is a maximum of 2 ft in depth and the surface layer is a maximum of"6 inches in depth. Each layer should be graded as follows. Subsurface Layer With the relatively flat hydraulic gradient available in the SFCW, a media with large void spaces enhances the movement of the wastewater through the SFCW. To achieve adequate void space, it is recommended that media size be 2-5 inches [50.8-127 mm] in diameter with a gradation as follows: Recommended Gradation 0% 10-20% 30-40% 50-80% 100% Retained Retained Retained Retained Retained 5. 4. 4. 3. 2. for SFCW Media 0 5 0 0 C in in in in in [125 [106 [100 [ 75 [ 50 mm] mm] mm] mm] mm] sieve sieve sieve sieve sieve The gradation should be verified at the origin and again at the construction site. The media should also be washed to remove any fine material before being placed in the SFCW. Surface Layer The six inch layer of rock on the surface of the SFCW should be comprised of 3/4 - 1\ inch [19.1-38.1 mm] washed stone to provide a base for the plants. Recommended Gradat 0% 40- 75% 85-100% 100% Retained Retained Retained Retained ion for Surface 1.5 1.0 3/4 1/2 in in in in [37.5 [25.0 [19.0 [12.5 Media mm] mm] mm] mm] \ sieve sieve sieve sieve 17 ------- Placement of Media The placement of the media is critical because of the need to maintain the void spaces within the SFCW. Media should be placed on a dry subgrade, preferably on top of a synthetic liner. The liner prevents the upward migration on soil particles from the subgrade. Heavy equipment should never be allowed on the filter media. ' This is to protect the void spaces from being reduced by compaction. Placement of the media should be accomplished by hand or by clamshell bucket. Sideslope/Substrate Protection Prevention of sideslope erosion and subgrade migration are critical to protecting the integrity of the SFCW. Installation of a synthetic liner is recommended. The liner should be installed to prevent seepage beneath the liner and on the sideslopes. Recirculation Capabilities Larger systems designed with conventional types of precedent treatment (other than septic tanks) should have the capability to recirculate the effluent from the SFCW back through the preceding process. Recirculation is required when the effluent does not meet the permit requirements; it is also important to be able to recirculate to augment low flow conditions so the filter will not dry out. Influent/Effluent Structures The influent structure should be designed to distribute the flow evenly across the width of the SFCW and be should placed near the surface of the stone media. The effluent structure should be placed near the bottom of the SFCW and the effluent discharge pipe should be designed to be adjustable to allow for control of the water level within the SFCW. Use of Plants While plants have been recommended for use in the SFCW, several systems without plants have achieved desired BOD and TSS reductions. Plants play a role in transferring oxygen to the wastewater in the filter though the actual amount of this transfer by type of plant is not known. Plants may also add to the organic load on the filter if leaves are allowed to drop on the surface and decay . This should be taken into consideration when the plant species are selected. Plants are sometimes used for aesthetic reasons. Some communities prefer to see the plants as a cover for the SFCW; some communities believe that the plants provide odor control. 18 ------- The plant species selected for use should be indigenous to the area so that it is tolerant of the local climate. Consult a horticulturist, botanist, county agent, or other authority on type of plants and timing of planting. Plant Spacing For comparatively large installations (up to 0.5 MGD) preceded by lagoon systems, initial research and design for the SFCW recommended plant spacing of about 2.5 feet on center. It has been found that such close spacing results in root growth that can reduce the available void space and contribute to ponding, i.e., flow that doesn't remain below the surface in the SFCW. The current recommendation is to place the plants 10 feet on center and stagger the rows. If more dense patterns are used, additional volume should be provided to account for loss of void space to the plant roots. Plant spacing for on-site SFCW systems can be much closer, on the order of 2.5 ft, because the hydraulic capability of these systems is generally greater than for larger systems. Typical Configurations Typical arrangements of SFWC systems with lagoon precedent treatment and septic tank precedent treatment are shown in Figures 3 and 4 respectively. 19 ------- TYPICAL CONFIGURATION FOR SCFW PLAN VIEW INFLUE C LAGOON CELL #1 ZD LAGOON CELL #2 LAGOON EFFLUENT NT TO LAGOON EN SFCW CELL #3 FLOW * SFCW CELL tfi FLOW 0 0 ° o SFCW CELL #1 O n ° , ?0 o 0 FLOW T TO SFCW 1 FIGURE 3 EFFLUENT TYPICAL SFCW CONFIGURATION FOR ON-SITE TREATMENT LONGITUDINAL SECTION SEPTIC TANKS T=0 INFLUENT SAMPLE EFFLUENT SAMPLE 18-24 IN A —i LARGE STONE (2.0 - 5.0 IN.) 6- OF SMALL STONE (0.75 - 1.5 IN.) 12 MIL PI PLASTIC LINER EFFLUENT DISCHARGE FIGURE 4 SECTION A-A 20 ------- CHAPTER IV - OPERATIONAL CONSIDERATIONS Although the SFCW system is very simplistic in design and operation, it cannot be left to operate by itself. While it is not a maintenance-free system, the maintenance required is primarily manual labor. With the exception of a recirculation pump that might be used only part time, there are few mechanical parts that must be maintained. At the same time, attention must also be given to the operation of the facility. The operational considerations or management practices will have an effect on the ability of the facility to maintain long term performance capabilities. The following management practices are necessary for the system to operate properly: 1. RECIRCULATION - the system should have the capability of recirculating the effluent for the SFCW back to either the preceding treatment or to the influent end of the SFCW. During times when the effluent from the system does not meet discharge permit effluent requirements or when additional flow is needed within the SFCW, the effluent flow should be recirculated. 2. PLANT MANAGEMENT - The SFCW facilities are designed for a specific root volume and a effort must be made to maintain this volume. Any increase in the root volume over that far which the SFCW _was designed will result in a decrease in the available void space. Thus, as plants grow and multiply, thinning of the plants is necessary to maintain the design root volume. Dead and dying material should also be removed to prevent decaying material from entering into the void spaces where it could aid in reducing the void space available within the stone media. Such material could also add to thr BOD and ammonia in the wastewater. All undesired extraneous vegetation should be removed periodically to prevent over growth of the stone media. Such growth could hamper the efficiency of the SFCW. In order to be effective in this system, some plants must be in their growth stage. These types of plants may require periodic trimming'to encourage growth. Whenever any trimming is done, all debris should be removed from the surface of the SFCW to prevent eventual migration into the stone media. 21 ------- BED MAINTENANCE - Where multiple SFCW cells are used in parallel, provisions should be designed into the system to allow for the removal of one or more of the cells from operation to perform maintenance (cleaning, media replacement, etc.). To extend the life of the SFCW, periodic back flooding of the stone media is recommended. This c^n be done with a high pressure hose inserted in the effluent collection line. Such a procedure will help in removing some of the detritus from the surface of the stone media and some of the solids deposition from the void spaces within the media. Control of liquid in the SFCW - If the SFCW is constructed with an adjustable effluent line as recommended, periodic adjustments may be necessary to maintain the proper liquid level in the stone media. For example, during periods of low flow, the effluent line should be adjusted to raise the level of the li~"id in the SFCW. At other times, the SFCW may need to be drained for maintenance. Experience has shown that if freezing temperatures occur, the SFCW should be flooded in order to protect the plant roots and to prevent freezing within the media which will cause the plants to be pushed upward out of the media. 22 ------- |