Evaluation of a Recirculating Sand Filter Followed by a Subsurface-Flow Constructed Wet la nd to Ac h i eve Den it r if i ca t i o n

Thomas J. Whitehill, P.E., Brian Tercha, and John F. Davis, Ph.D., P.E.

Abstract: This paper examines the denitrification capabilities of two recirculating sand filter (RSF)/subsurface-flow constructed wetland (SSFCW) systems in southeastern Pennsylvania, one serving a grocery store and the other a summer camp. The systems achieved denitrified effluent with very l i t t le operator attention, making this technology combination well-suited for low-flow applications.

Recirculating sand filter (RSF)/subsurFace-flow constructed wetland (SSFCW) wastewater treatment system installed at Mussefs Market i n Lancaster County, Pennsylvania. RSF beds are shown i n the foreground and the SSFCW i n the background.

A recirculating sand filter (RSF)/sub- surface-flow constructed wetland (SSFCW) wastewater treatment system has been applied in two different small- flow system applications in southeastern Pennsylvania requiring denitrification. A flow diagram of this process is shown in Figure 1. Achieving reliable denitrifica- tion has been difficult for small-flow on- site disposal systems due mainly to the flow and waste strength variations these systems encounter. The RSF/SSFCW system has the advantage of physically separating the aerobic and anoxic phas- es of biological treatment required to achieve denitrification, which is one of the most difficult aspects of achieving denitrification in activated sludge processes. The subsurface wetland is capable of maintaining anoxic condi- tions without emitting offensive odors.

Denitrification Nitrogen plays a significant role in

the quality of water in agricultural areas such as Lancaster County, Penn- sylvania, the Chesapeake Bay Water- shed, and the Midwest states. Nitro- gen contaminated groundwater can affect the safety of water for human consumption and degrade the quality of natural waters. Nitrate concentra- tions above 10 mg/L-N have been re- ported to cause the condition methe- moglobinemia in sensitive individuals. Areas experiencing heavy nitrogen in- puts to surface or ground waters are currently required to remove nitrogen during the wastewater treatment process to prevent further degrada- tion of water quality.

Nitrogen can be removed from wastewater using a two-stage biological

nitrification-denitrification process. The first stage, nitrification, uses aerobic bacteria to convert ammonia-nitrogen to nitrite-nitrogen (Nitrosomonas bac- teria) and then to nitrate nitrogen (Ni- trobacter bacteria). Nitrification oc- curs only after the carbonaceous bio- chemical oxygen demand (CBOD) of the wastewater has been sufficiently depleted. These two steps are sum- marized below in Equations 1 and 2:

Equation 1 (nitrosomonas)

NH4, +3/2 0 2 3 N02- t 2H+ + H20

Equation 2 (nitrobacter)

N02-- +’h 0 2 +Nos-

-. -.

-

This method uses fecal Streptococcus (including the Enterococci) and/or E. coli and patterns of antibiotic resistance for separation of sources. The premise is that human fecal bacteria will have the greatest resistance to antibiotics and that domestic and wildlife animal fecal bacteria will have significantly less resistance (but still different) to the battery of antibiotics and concentrations used. Most investigators are testing each isolate on 30 to 70+ antibiotic concentrations. Fecal bacteria are grown in wells in microtiter trays and then replica-plated onto a series of agar plates, each containing one specific antibiotic concentration. Forty-eight cultures can be transferred to each agar plate simultaneously with a stainless steel replicator. After incubation, each isolate is scored for growth or no growth on each plate, and a rcsistance pattern emerges that can be used in source differentiation.

F-Specific (F+ or FRNA) Coliphage

iphages can be differentiated using RNA coliphages, and those predom-

minating in animals (groups I and IV).

coliphage isolates. However, there is a problem

ilus synthesis ceases, The coliphage of F-pilus E. coli, and is not found in

FRNA coliphages may be able to re- does not replicate in the environment, only in the

Sterols or Fatty Acid Analysis

branes verses those in other anim no published reports of its use in

y under development and there are

Nutritional Patterns This technique is based on differences among bacteria in their use of a wide range of carbon and nitrogen sources for energy and growth. This method works well in the laboratory. However, there are many environmental factors in a watershed that can affect bacterial nutrient require- ments that may make this method impractical for field determination. The BIOLOG system al- lows the user to rapidly perform, score, and tabulate 96 carbon source utilization tests per isolate and is widely used in the medical field for microbial identification. Another modification of the nutritional pattern concept is the use of human-specific (sorbitol-fermenting) bifidiobacteria as in- dicators of nonpoint source human fecal pollution.

Fecal Bacteria Ratios This procedure is based on the ratios (presence and numbers) of many different types of stom- ach and intestinal bacteria, not just fecal coliform bacteria, to develop a ratio coefficient that could be useful in source identification. While the traditional fecal coliform-fecal streptococcus ratio is no longer considered reliable for accurate source identification, ratios may still be useful as a general indicator of human verses nonhuman fecal bacterial contamination, and the ratio concept could perhaps be found more reliable i f othcr microbes were used in developing the ra-

The second stage, denitrifi- cation, uses numerous genera of naturally occurring faculta- tive bacteria to convert nitrate- nitrogen to gaseous-nitrogen as shown in Equation 3. Denitrifi- cation requires dissolved oxy- gen concentrations below 1 mg/L and an adequate carbon source to support the biological respiration that drives this process. The gaseous nitrogen i s then liberated to the atmos- phere completing the overall denitrification process.

40.0 T Flnat E f 9 bent

Equation 3 NOg 3 NO2- 3 NO+ 3 N20+ +N2+

(Metcalf and Eddy, 1991)

RSFs The recirculating sand filter was first

introduced in the 1970s by Michael H. Hines in Illinois. The RSF was a direct descendent of the intermittent sand fil- ter which dates back to the mid-l 800s. The first RSFs consisted of a septic tank, recirculation tank, and a sand fil- ter. The contents of the recirculation tank were intermittently dosed over the sand filter throughout the day, while a flow splitter allowed the effluent to be either returned to the recirculation tank or diverted to a disposal tank (Crites and Tchobanoglous, 1998).

More recent versions of the RSF use fine gravel in place of the sand media. The gravel media typically has an effective size of 2.00 to 3.00 mm, with a uniformity coefficient of >2. The use of a gravel media has made the practice of periodically removing the top layer of sand unnecessary, thus re- ducing the maintenance requirements for this system. Following the replace- ment of the sand with fine gravel, these filters are referred to by several descriptive names. This article retains the original nomcnclaturc of rccirculat- ing sand filter even though the sand has been replaced with fine gravel.

Wetland Systems Natural wetlands have most likely

been used for wastewater disposal since the beginning of wastewater collection. The earliest documented use dates back to the early 1900s (EPA, 2000). Constructed wetland use began in Eu- rope in the 1950s and spread to the U.S. by the 1960s. The subsurface-flow wetland design used in this project was most likely introduced in 1980s (EPA,

2000). A typi- cal SSFCW treats water by allowing the waste- water to flow laterally through the media and contact the microbiologi- cal agents re- sponsible for nitrification

Influent (Screened Septic Tank Effluent) Characteristics

I I ( ) = sample count

and denitrification (Crites and Tchobanoglous, 1998).

Site Descriptions The first site to employ the

RSF/SSFCW treatment system in this study, Black Rock Retreat, is a Christian camp and retreat center offering motel lodging in the form of two lodging wings with a total of 61 rooms, a small motel with 8 rooms and a dozen bunk cabins. The 100-acre retreat is located in southern Lancaster County, Pennsyl- vania, in a rural and agricultural setting. The surrounding agricultural activities and site geology have contributed to the elevated nitrate-nitrogen present in the groundwater at this site.

Because the background ground- water nitrate levels exceed the drinking water standard of 10 mg/L nitrate-N, the retreat was required by the Penn- sylvania Department of Environmental Protection (PADEP) to use denitrifica- tion technology on new flows created during an expansion project in 1998. The additional flows created during the expansion were approximately equal to the preexpansion flow of 6,000 gal- lons per day (gpd), resulting in a total wastewater flow of 12,000 gpd (maxi- mum daily flow).

The second site, Musser's Market, is a 50,000-ft2 grocery store construct- ed in late 2002 and placed in opera- tion in February 2003. This store has an on-site meat packing operation that contributes significantly to the waste organic load. The waste stream pro- duced by Musser's Market has a very high CBOD compared to that of Black Rock, although the total nitrogen con- centration is similar (see influent char- acteristics in Table 1). The groundwa- ter nitrate levels in the vicinity of this store showed elevated concentrations, which made denitrification of their wastewater flow necessary.

Materials and Methods The design parameters for the RSF

system were taken from Metcalf and Eddy (1 991 ). The wetland hydraulic residence time was derived using data from bench scale studies on denitrifi- cation rates in subsurface-flow wet- lands. The septic tank volume require- ments for primary treatment were taken from Pennsylvania Code Title 25, Chapter 73, Standards for Sewage Disposal Facilities. Analytical results were provided by two laboratories, each certified by the PADEP for envi- ronmental testing.

0

Campers at Black Rock Retreat. I n the summer months, wastewater flows from the retreat can equal as much as 12,000 gallons per day from 100 campers plus lodging guests and staff.

The methodology used at each site is listed in Table 2. Methods pre- ceded by SM- refer to methods de- scribed in Standard Methods for the Examination of Water and Wastewater, 20th Edition, while methods preceded by EPA- refer to methods described in the EPA manual Methods for Chemi- cal Analysis of Water and Wastes (EPA, 1983). The United States Geo- logic Survey method, USGS-1-3675, is a modified total suspended solids test used by the PADEP laboratory for TSS analyses. All samples were grab-type samples. Both wastewater systems de- scribed in this article were designed and permitted through the PADEP by Whitehill Consulting Engineers.

Processes The innovative technology being

evaluated at Black Rock Retreat (and later tested at Musser's Market) in Lancaster County, Pennsylvania, is a RSF/SSFCW series treatment system. The following gives a detailed descrip- tion of the processes involved in this system.

Primary Treatment Accomplished by septic tanks or

other settling tank configuration of ade- quate design to provide reliable and ef- ficient solids removal prior to screening of the primary effluent. The design hy- draulic retention time for primary solids removal is two days at maximum design flow rates. Solid particles from the wastewater flow must be separated

from the main flow and subsequently disposed of to prevent their entry into the RSF. Solids could cause the filter to clog and prematurely fail or necessitate excessive maintenance of the filter and dosing laterals.

Primary Effluent Screening Accomplished by a number of

treatment options to further reduce the solids concentration in the pri- mary effluent prior to discharge to the RSF recirculation tank, primary efflu- ent screening is a necessary treatment step to prevent clogging of the dosing pipe orifices and/or the media in the RSF. The effective screen opening size should be equal or less than the dos- ing lateral orifice diameter.

RSF This filter typically consists of fine

gravel media to a depth of 24" to 30" (effective size 2.9 to 3.0 mm). Note: Ef- fective size, d10, is de- fined as the diameter of a particle of filter media of which 10 percent of the media is smaller by weight and is typically determined by a sieve analysis).

The fine gravel media supports the bacterial "fixed film" that treats the wastewater by reducing the organic strength (BOD) and converting the ammonia in the wastewater to nitrate -N in the ni- trification process previously de- scribed. The fixed film consists of a variety of species of bacteria, which are present in the waste- water being treated and attach to the filter media. The wastewater is treated as it flows over the fixed bacterial film which consumes the CBOD of the wastewater and converts the ammonia nitrogen to nitrate. The 24- to 30-inch layer of fine gravel is supported by 9- to 12-inch layer of graded coarse gravel with embedded underdrain laterals, which serve to drain the treated wastewater back to the re- circulation tank. Aerobic condi- tions are maintained in the filter

media by pulse-dosing the wastewater from the recirculation tank to the media via programmed cycling of the dosing pumps located in the recircula- tion tank.

The dosing pumps apply the wastewater through the dosing later- als for 1 to 3 minutes every 20 to 30 minutes. The duration and frequency of dosing i s dependent on influent wastewater strength, influent flow rate, temperature, and effluent limits for CBOD and total nitrogen concen- trations. After dosing, the wastewater drains from the media and is divided between two flow paths via a flow splitter valve installed in the recircula- tion tank. A maximum of 20 percent of the RSF effluent is diverted to the wetland for subsequent treatment with the remaining flow reintroduced to the recirculation tank. This configu- ration results in a minimum 5:l recir- culation ratio. After draining, air occu- pies the voids between the filter

media providing for oxygen transfer to the fixed biological film to maintain aerobic conditions.

Significant denitrification occurs within the recirculation tank of the RSF treatment process due to the mix- ing of the screened septic tank influ- ent and RSF effluent recirculation flow prior to dosing to the RSF bed. The ef- ficiency of denitrification in this phase of treatment ranged from about 45 percent at Black Rock Retreat to over 84 percent at Musser's Market. The higher organic content of the Musser's Market waste stream was the main cause of the higher rate of denitrifica- tion in the RSF recirculation tank. The Musser's Market application of this technology is able to meet their dis- charge limits for total-N without a car- bon source feed to the wetland.

SSFCW The SSFCW provides the anoxic

environment (with external carbon source) necessary to convert the ni- trate-N in the RSF effluent to nitrogen gas thereby completing the denitrifica- tion process. The wetland consists of inlet distribution piping, an impervious liner to prevent groundwater contami- nation, 30 to 60inches of grav- el media (effective size - 8") and outlet piping to collect the denitrified wastewater and deliv- er it to an effluent collection tank for subsurface disposal or, alternatively, to an approved dis- infection process and stream dis- charge.

A bypass flow of 5 to 25 percent of the septic tank screened effluent flow was ini- tially diverted around the RSF di- rectly to the wetland to provide the carbon source to achieve denitrification. The bypass ratio depends upon the influent wastewater strength and the ef- fluent limits for total nitrogen concentration. The bypass flow has a dual purpose as it provides a carbon source to assist in the denitrification treatment step, and it also accelerates the transi-

carbon source being fed to the bacteria in the SSFCW. In March of 2002, the bypass flow at Black Rock was discon- tinued and replaced with a flow-paced methanol feed. The methanol is fed into the portion of the RSF effluent flow di- verted to the wetland to act as the car- bon source. After the installation of the methanol feed, the constituent concen- trations in the final effluent became much more acceptable and predictable, particularly with respect to total-N.

The waste characteristics at Muss- er's Market resulted in a much higher rate of denitrification in the RSF recircu- lation tank than was originally noted at Black Rock Retreat. Although config- ured the same, the RSF/SSFCW system at Musser's Market does not require a supple- mental carbon source feed to the wetland cells to meet denitrifica- tion requirements.

Performance Tables 1, 3, and 4

show the difference in average concentration of selected wastewater

Musser's Market showed significantly higher organic strength compared to Black Rock.

Table 3 shows analytical results from the RSF effluent at the Black Rock Retreat facility. These data are presented to show the relative efficien- cy of CBOD and total-N reduction through this portion of the treatment process. The RSF showed an average total-N reduction of 44.5 percent. Total-N reduction in the RSF i s accom- plished by creating anoxic zones in the RSF recirculation tank and the efficien- cy of this process is directly related to the influent CBOD and the hydraulic residence time of the recirculation tank. Nitrite concentrations were

Black Rock Retreat RSF Effluent Results.

RSF Effluent Average Concentration (mg/l) CBOD5 4.1 ( 1 6 )

N-Total 44.7 ( 8 7 )

NO3-N 35.9 ( 8 7 )

N02-N 5.3 ( 8 7 ) *0 .4 ( 7 4 )

NH3-N 3.6 ( 8 7 )

ss 6.0 ( 1 4 )

( ) = sample count

Final Effluent Sample Results

Average Concentration (mg/L, except fecal coli. as geometric mean/100 mL.) Location

Dates of operation

Treatment Process

N-Total

NO3-N

CBOD5

N02-N

NH3-N

TKN ss

Fecal Coliform

BRR

11/01 -7/03

RSF/wetland 5.7 ( 8 2 )

21.7 ( 8 6 )

7.1 (88)

2.7 (88)

9.1 (88)

12.2 (88)

9.3 ( 8 2 )

220 ( 3 8 )

MM

8/02 - 8/03

RSF/wetland 9.0 ( 1 3 )

12.6 ( 2 5 )

8.0 ( 2 5 )

0.2 ( 2 5 )

2.5 ( 2 6 )

4.4 ( 2 6 )

7.3 ( 1 3 )

6.4 ( 1 3 )

BRR = Black Rock Retreat: MM = Musser's Market: SBR = Sequential Batch Reactor

( ) = sample count

tion from aerobic (dissolved oxygen concentration > 1 mg/L) to anoxic con- ditions (dissolved oxygen concentra- tion < 1 mg/L) in the RSF effluent as it enters the SSFCW. After adequate tests had been run on the final effluent, it was found that the bypass flow was not yielding high-quality results. it seemed as though there was not an adequate

constituents before and after the instal- lation of the RSF/SSFCW. All measure- ments are in mg/L. Table 1 shows the influent (screened septic tank effluent) characteristics at both the Black Rock Retreat and Musser's Market locations. The influent waste characteristics at Black Rock were typical of domestic waste flows. The influent waste at

skewed due to high nitrites in the RSF effluent during the start-up phase of this system, which began in Novem- ber 2001. The average nitrite concen- tration of the RSF effluent with the start-up data removed is shown in Table 3 with the asterisk. The estab- lishment of nitrifying bacteria was slowed by the cooler temperatures 0

encountered during start-up; however, once established, the nitrifying bacteria were not ad- versely affected by winter tem- peratures.

Current design criteria rec- ommends a hydraulic residence time of 0.5 to 1 .O days based upon design flow of the system for sizing the recirculation tanks. Where denitrification effi- ciency is important, the higher residence time should be used to maximize denitrification in this phase of treatment. Denitri- fication efficiency was further improved by incorporating a center wall baffle into the de- sign of the RSF recirculation tanks to minimize the effects of short-circuiting.

Black Rock Retreat Flow Data (Gallons per Day).

Jan. '02 Feb. '02 Mar. '02 Apr.'O2 May '02 June'O2 July '02 Ave. 2,472 3,221 4,999 3,919 4,071 4,265 6,032

Max. 8,037 12,052 13,500 9,801 11,818 13,337 9,551

Stdev. 2,015 2,794 3,412 2,450 2,837 2,999 1,940

Aug. '02 Sep. '02 013. '02 Nov. '02 Dec. '02 Ave. 5,015 4,290 4,145 4,517 3,422

Max. 9,482 11,043 8,695 12,100 12,155

Stdev. 2,363 2,681 2,295 3,216 2,979

Jan. '03 Feb. '03 Mar. '03 Apr.'O3 May '03 June'03 July '03 Ave. 2,800 4,092 5,736 4,418 3,905 4,865 7,144

Max. 10,465 14,700 14,730 10,365 10,575 12,605 10,110

Stdev. 2,629 3,525 3,462 2,937 2,764 3,357 1,956

The Black Rock Retreat final efflu- ent data are presented in Table 4 in three separate sets by date. The data set from November 1998 through No- vember 2001 represents performance data from the 6,000 gpd design flow sequential batch reactor (SBR) original- ly installed to achieve denitrification at this site. The poor performance of this system to meet established effluent limits resulted in it being replaced by the RSF/SSFCW treatment system in October, 2001. The RSF/- SSFCW sys- tem was placed in operation October 3 1,2001, and effluent sampling began November 2001. The data set from No- vember 2001 through July 2003 repre- sents average values of all data collect- ed from the 12,000 gpd design flow RSF/SSFCW treatment system to date. Finally, the data from March 2003 through July 2003 represents the per- formance data collected after the methanol feed system was stabilized and a malfunctioning automatic distrib- uting valve in the RSF dosing plumbing was repaired.

The Musser's Market data present- ed in Table 4 includes all data to date for the 8,000 gpd design flow RSF/SSF wetland system installed July 2002 at this site. The average total-N value of 12.6 for the Musser's Market system is somewhat skewed by the February 2003 monthly average of 37.0 mg/L total-N. The average total-N concentra- tion is 10.5 mg/L without inclusion of the February 2003 data. The effluent quality of the Musser's Market system during the winter months of 2003 was impaired by the colder than normal temperatures experienced which result- ed in wetland effluent temperatures

Musser's Market Flow Data (Gallons per Day) I

Aug. '02 Sep. '02 Oct. '02 Nov. '02 Dec. '02 Jan. '03 I Ave. 1,507 1,785 2,019 1,527 1,568 1,897

Max. 3,732 2,917 3,908 2,652 1,792 2,818

Stdev. 560 508 575 498 304 341

Feb. '03 Mar. '03 Apr.'03 May '03 June'03 July '03 I Ave. 2,408 2,267 2,442 2,336 2,578 2,124

Max. 2,818 2,643 3,388 2,807 2,876 2,243

Stdev. 248 260 818 349 288 102

below 2oC. The practice of bypassing flow around the RSF to the wetland for a denitrification carbon source added to the total-N loading onto the wetland. Future operation at this facili- ty will be to operate without a bypass flow and add the carbon source to the wetland through a flow-paced methanol feed if required to meet dis- charge total-N limits. Methanol does not contribute to the total-N loading to the wetland and provides a more easily metabolized carbon source.

Although not reported, tempera- ture was monitored through the treat- ment process to determine its effect on system performance. The minimum temperature of the RSF effluent at ei- ther facility was measured at 6oC. At the minimum RSF effluent tempera- tures noted, the nitrification process was slightly affected but not to the point of affecting the systems ability to meet denitrification performance re- quirements. The wetland temperatures however approached IOC, which did adversely affect the denitrification process. In very cold climates, the non- vegetated wetland may benefit from an 18- to 24-inch soil cover for insula-

tion. The impact of this soil insulation cover is planned for testing at the Black Rock Retreat facility during the winter of 2004 to 2005.

data for the Black Rock Retreat and Musser's Market systems respectively. The flow pattern for Black Rock is high- ly variable and does affect system per- formance. Based upon an analysis of water supply meter readings taken at Black Rock Retreat, water demand, and hence, wastewater flows, are generated unevenly with approximately 25 per- cent of the total daily flow occurring between the hours of 6:OO and 9:00 a.m. Water demand is essentially non- existent between the hours of mid- night and 6:OO a.m., resulting in an 18-hour runoff period without flow equalization. The result of the flow patterns at Black Rock Retreat is peri- odic hydraulic overloads when daily flows approach design flows and sub- sequent ammonia-N breakthrough in the RSF beds.

Although this system is currently performing well and meeting its efflu- ent discharge limits, it is planned to in- stall influent flow equalization at this

Tables 5 and 6 summarize the flow

site to reduce diurnal flow variability to the RSF and further improve per- formance. Influent flow equalization is incorporated at the Musser’s Market system and appears to have enhanced the performance of the RSF/SSFCW wetland system there. All flow data presented are final effluent metered flows and includes rainfall onto the RSF and wetland cells, which can con- tribute significantly to the metered to- tals. It is primarily the rainfall contribu- tion that accounts for the variability shown in the Musser’s Market data.

Benefits The most frequently used reactors

to denitrify wastewater in larger flow in- stallations are activated sludge process- es either of conventional design or se- quential batch reactor mode. These methods involve inducing dissolved air into the wastewater or mixed liquor to efficiently reduce the organic strength (CBOD) of the waste and simultane- ously nitrify the wastewater through aerobic biological processes. The aera- tion rate is then reduced and endoge- nous bacterial respiration causes the dissolved oxygen level to drop below 1 mg/L permitting the denitrification step to proceed.

The success of activated sludge sys- tems to achieve nitrification/-denitrifica- tion is highly dependent on monitoring and controlling dissolved oxygen levels and mixed liquor suspended solids con- centration during each phase of treat- ment. These systems work reasonably well in larger facilities that employ full- time operators and automated monitor- ing equipment. Small community, com- mercial, and on-lot wastewater treat- ment systems attempting to achieve denitrification generally face a more dif- ficult task in that the waste quality and quantity can be highly variable, and these systems often do not employ full- time certified operators familiar with the denitrification process or sophisti- cated process control equipment.

The RSF/SSFCW denitrification treatment approach offers significant advantages over activated sludge sys- tems for small flow applications. Nitri- fication and denitrification are carried out in two separate processes that are independently designed and opti- mized for CBOD removal and nitrifi- cation (RSF) and denitrification (wet- land). This is achieved by adjusting the dosing frequency and duration to the RSF, and by adjusting the methanol feed rate to the SSFCW in response

to influent waste characteristics. The energy requirements for the RSF/SSFCW system offer significant energy savings compared to activated sludge systems. The RSF/SSFCW sys- tems also require significantly less op- erator attention than activated sludge systems due to their simple and pas- sive design.

In summary, the RSF/SSFCW sys- tem provides a reliable and cost effec- tive option for smaller wastewater sys- tems that need to denitrify effluent prior to final disposal. Applications of RSF/SSFCW systems are likely to in- crease as denitrification becomes re- quired through the implementation of water quality standards within water- sheds of high quality streams.

Conclusions The purpose of this project was to

evaluate the capability of the RSF/- SSFCW technology to denitrify waste- water in two small flow applications. The test results from Black Rock Re- treat and Musser’s Market installations support this technology’s capability to achieve that treatment goal. This proj- ect proved the value of the recirculat- ing sand filter/wetland technology to achieve a denitrified effluent with very little operator attention. The separate aerobic (RSF) and anoxic (wetland) processes provide a treatment train which is much easier to optimize for denitrification than the activated sludge systems currently available. The stable operating conditions and low opera- tion and maintenance requirements of this technology make it an excellent option for low flow applications where denitrification is required.

An unexpected benefit observed with this treatment train was the consis- tent level of inactivation of fecal col- iform bacteria in the final effluent sam- ples from both systems. This ability to achieve disinfection levels of fecal col- iform bacterial inactivation through the subsurface flow wetland may prove valuable in applications of stream dis- charge where residual chlorine concen- trations could prove detrimental to aquatic life. Additionally, the high quali- ty effluent this treatment system can produce makes it an excellent candi- date for evaluation in water re- use/reclamation applications, particular- ly when configured without a carbon source feed into the wetland.

Acknowledgements We wish to express our apprecia-

tion for the input and advice received from Grant Denn and Terry Bounds at Orenco, Inc. in the design and opera- tion of the recirculating sand filter por- tion of the overall process. Don Brown of the U.S. EPA was very helpful in pro- viding information on wetland design and reaction rate kinetics for denitrifi- cation within a subsurface-flow con- structed wetland. Finally, and most im- portantly, we would like to thank Jim Miller and Ed Corriveau from the Southcentral Regional Office and Pete Slack and Tom Franklin from the Cen- tral Office of the PA DEP for believing in the value of this project and provid- ing funding to install and monitor it at the Black Rock Retreat location.6DDI

References Metcalf and Eddy (eds). 1991. Wastewater

Engineering Treatment, Disposal and Reuse. Third Ed. Boston: McCraw-Hill, Inc.

Crites and Tchobanoglous. 1998. Small and Decentralized Wastewater Management Systems. Boston: McGraw-Hill, Inc.

U.S. Environmental Protection Agency. 2000. Constructed Wetlands Treatment of Mu- nicipal Wastewaters. Cincinnati. EPA/625/R-99/010.

Thomas J. Whitehill, P.E., i s president of Whitehill Consulting En- gineers (www.whitehil1- consultingeng.net) and can be reached via email at twhitehillQ epix.net (71 7) 548-3557.

Brian Tercha i s a student intern a Whitehill Consulting Engineers entering his senior year at Millersville University as an Earth Science major and can be reached at bmtercha8hotmaii.com.

John F. Davis, Ph.D., P.E., is an Associate Professor in the Department of Civil Engineering at Widener University and can be reached at j0hn.f. davisQwidener.edu.

Ed Winant, P.E.

What i s a "bio"-filter?

What does it do? Why would I install one instead qf a drainfield?

/

d

for biological filter, is a secondary treat- onsite wastewater. Here, secondary

process. Primary treatment reaction. Secondary

a biological

Filtration is one of the more common cal treatment processes. Filters are com constructed using sand, gra thetic material. These synth foam, fabric, or plastic are under the generic title "bi

where bacteria living in t Biological treatment is a nat

ent that passes by. Bacterialas they live and eat, grow and reproduce, fo rdng a slimy, black layer called a biomat on the filter.

This biomat, while necessary for treatment, can pose problems. If it grows too thick, it will clog the pore spaces in the filter, and the efflu-

twi l l pond on top. This introduces the critical rs: surface area to volume ratio.

tio shows how much area is available for live on as opposed to the

lume the filter takes up. using foam or plastic, provide a surface area to volume. Foam has

cture with lots of pore spaces. formed into spheres with lots of

interior brackg to provide more space for bacte- ria to attach. fhus, biofilters can support a large amount of bacterial growth while still allowing water to filter past. The offset to this is a higher cost for the filter media than sand or gravel. The other main difference is maintenance. With a biofilter, a backwash device should be included, that will force clean water upwards through the filter, scouring off the biomat to clean the filter

1-1 I I I

Biofi I ter Module

Treated effluent weeps from the base of the modules or is collected

for disposal by other methods I Septic Tank with Filter I I

Pump Tank