Water reuse for sludge management and wetland habitat

e

Wat. Sci. Tech. Vol. 33, No. 10-11, pp. 213-219,1996. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved. 0273-1223/96 $15'00 + 0·00

Pergamon

PH: S0273-1223(96)00422-2

WATER REUSE FOR SLUDGE MANAGEMENT AND WETLAND HABITAT Sherwood Reed*, Susan Parten**, Gary Matzen*** and Randy Pohrent

* Environmental Engineering Consultants, 50 Butternut Road. Norwich. VT 050. USA ** Community and Environmental Services. 2101 S.lH-35. Suite 400. Austin, TX 78741, USA *** CH2M-Hil/, 8911 Capital of Texas Highway, Suite 1110, Austin, TX 78759, USA t City of Austin Water/Wastewater Utility, 625 E. 10th St.. Austin 78701, USA

ABSTRACT The Hornsby Bend Sludge Management Facility provides centralized stabilization and dewatering for the sludges from several wastewater treatment plants serving the City of Austin, TX. This facility has been utilizing water reuse for a number of years via land application of the treated leachate and run off from the site, on crop land. The treatment sequence included aerated and facultative lagoons and a large greenhouse structure containing water hyacinths. Plans are under development to modify and upgrade the sludge treatment facilities, these will include mechanical dewatering. The reuse of this treated filtrate as wash water in the facility is intended for future operations. This paper describes a conceptual plan for treatment of this filtrate by conversion of one of the existing lagoons to a wetland. A significant portion of this wetland will be developed for optimum habitat value. Public access to this wetland can be provided for observation of birds and other wildlife. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS Water reuse; sludge management; sludge dewatering; filtrate treatment; wetlands; habitat value; wastewater treatment.

INTRODUCTION The objective of this effort was to develop a cost effective method for water reuse at the Hornsby Bend Sludge Management Facility in Austin, Texas. The intended reuse is wash water for the mechanical sludge dewatering operations and land application of any excess. The source water for this effort would be filtrate from the thickening and mechanical dewatering operations. A major component in the treatment sequence for this water would be a constructed wetland. A portion of the wetland would be developed for enhanced habitat value.

BACKGROUND The Hornsby Bend Sludge Management Facility stabilizes and dewaters the sludge from the City of Austin's Govalle, Walnut Creek, and South Austin Regional waste water treatment plants. The combined treatment capacity of these three waste water treatment plants is approximately 27,000 m3/d average flow. 213

214

S. REED ef al.

These three systems pump all of their untreated primary and secondary sludges to the Hornsby Bend facility; the volume of sludge delivered is approximately 50 mtld; at three percent solids that would be equivalent to 1700 m3 of liquid sludge per day. The existing sludge management operations at Hornsby Bend include mechanical sludge thickening, anaerobic sludge digestion, 10 ha of drying beds for sludge dewatering, and composting for the dewatered sludge. The liquid side streams from the thickening and drying bed operations are combined with storm water runoff and discharged to a series of lagoons for treatment. In addition to these liquid side streams excess digested sludge from the anaerobic digestors is also occasionally diverted to the first lagoon cell in the series. Originally, the effluent from these ponds was disinfected and then discharged to the Colorado River. This effluent did not always meet the discharge requirements for the facility. Water hyacinths were introduced into the 1.2 ha chlorine contact basin and served as a seasonal upgrade for the treatment process for several years. The Austin area experiences freezing weather during the winter months and this condition damages the hyacinth plants and reduces their treatment effectiveness. A new plastic greenhouse structure was erected to correct this problem and permit continuous year-round hyacinth treatment. The large greenhouse structure covers a total area of 5 ha. It contained three shallow basins. The center basin has a surface area of 0.64 ha, and the two outer basins have a surface area of 0.48 ha. All three basins are 265 m long, their depth varies from 0.9 m at the influent end to 1.5 m at the effluent end. Until 1989 effluent from this facility was discharged to the Colorado River, the discharge requirements were 30 mg/L BOD and 90 mg/L TSS. Performance of the hyacinth system was not consistent, the effluent TSS was usually satisfactory but the BOD removal of the facility was erratic, due primarily to non uniform flow and significant variations in organic loading. Since 1989 the Hornsby Bend facility has utilized water reuse for this treated liquid side stream (US EPA, 1988). The effluent from the greenhouse has been used for irrigation of 65 ha of feed and fodder crops on an on-site City owned farm. The farming operations were performed by a contracted farmer who returned 21 percent of his total receipts from the operation to the City. The sludge solids processed at the facility is also reused via composting and distribution or land application.

PLANNED IMPROVEMENTS Basic changes in the Hornsby Bend facility are proposed to increase capacity, reduce odors, and increase reliability of the operations. Major components in those changes involve installation of mechanical dewatering equipment and closure of the 10 ha of drying basins and two of the lagoons which formally received liquid side streams and some digested sludge. These changes will increase both the volume and the strength of the liquid side streams requiring treatment since they will be composed of filtrate from the thickening and dewatering operations as compared to the former drying bed decant and stormwater runoff. Since the Hornsby Bend facility can no longer discharge any effluent to the Colorado River it is necessary to develop methods for effluent reuse. The anticipated future reuse opportunities include wash water for the thickening and dewatering operations and land application for agricultural production. The total volume of filtrate requiring treatment is expected to be about 7600 m3/d. Wash water uses will require about 2700 m3/d with the balance going to land application. The expected filtrate characteristics prior to treatment are listed in Table 1. The water quality goals for the treated sludge filtrate are given in Table 2. The BOD and TSS levels were considered to be the limiting parameters for wash water reuse, with ammonia levels below 10 mg/L as a secondary goal. A number of alternatives and combinations were considered for treatment of this wastewater stream. All of the possibilities included the use of adjacent treatment facilities which were originally constructed to treat the wastewater from Bergstrom Air Force Base. This facility was designed as an activated sludge process (without primary clarification) but has been abandoned since the Air Force

Sludge management and wetland habitat

215

TABLE 1. EXPECTED SLUDGE FILTRATE CHARACTERISTICS Characteristic

Quantity

Flow BODs TSS NH 3/NH 4 Temperature (minimum)

7570 m3/d 1000 mg/L 800 mg/L 60 mg/L 16°C

Base ceased operation. One alternative considered conversion of the Bergstrom facility to a chemically enhanced primary clarification treatment system. This was projected to produce (CH2MHill, 1995) a primary effluent with 600 mg/L BOD and 160 mg/L TSS. Such effluent would not be suitable for direct application to a wetland so additional treatment would be necessary. This would require retention and modification of the two lagoon systems for preliminary treatment prior to the wetland component. Although technically feasible, this alternative was not cost effective and was eliminated. TABLE 2. WATER QUALITY GOALS FOR THE TREATED FILTRATE Characteristic

Quantity 15 mg/L 15 mg/L < 10 mg/L

The selected cost effective alternative proposes addition of primary clarification to the Bergstrom facility and then the utilization of this plant as an activated sludge process to produce effluent of acceptable quality for direct application to a wetland component. The expected effluent quality from this primary clarification plus activated sludge process (CH2MHill, 1995) are given in Table 3. TABLE 3. EFFLUENT CHARACTERISTICS FROM ACTIVATED SLUDGE TREATMENT Parameter m3/d Influent Effluent % Reduction

Flow Rate mg/L

BOD mg/L

TSS mg/L

7570 7570

1000 52 95

800 30 96

NH 3/NH4 mg/L 60 10 83

WETLAND DESIGN Constructed wetlands are finding increasing use, world-wide, for treatment of a variety of wastewaters. It is estimated that there are at least 1500 systems treating municipal wastewaters in a variety of countries (Knight, Kadlec, Reed, 1993). There are two types of constructed wetlands in use, both types model the basic features present in a natural marsh. The first type, termed a free water surface (FWS) wetland, has the water surface exposed to the atmosphere. The bed contains emergent aquatic vegetation, a layer of suitable soil to serve as a rooting media, a liner if necessary to protect the groundwater, and appropriate inlet and outlet structures. The water depth can range from a few centimeters to 0.8 m or more depending on the purpose of the wetland. The second type of wetland is called a subsurface flow (SF) wetland. In this case the excavated basin is filled with a porous media, such as gravel, and the water level is maintained below the top of the gravel. The same species of vegetation are used in both types of wetlands; in the SF

S. REED el ai.

216

type the vegetation is planted in the upper part of the gravel. The depth of the gravel media is typically OJ to 0.6 m. There are several advantages to the SF type of wetland. The biological reactions occurring in both types of wetlands are believed due to attached growth organisms. Since the gravel media has more surface area than the FWS wetland, the gravel bed will have a higher reaction rate and therefore can be smaller in size. Since the water surface is not exposed there are no public exposure risks, and mosquitoes are not a problem with the SF type. The SF type also provides greater thermal protection in colder climates. The major disadvantage of the SF type is the relatively high cost to procure and place the gravel in the bed. Cost comparisons have shown that the SF wetland is not usually economically competitive with the FWS type at design flow rates greater than 400 m3/d. In many cases the other advantages of the SF concept are more important than costs and it is often selected for schools, parks, and public and commercial buildings. These advantages are not a significant factor in the Hornsby Bend application and since the design flow is 7570 m3/d a FWS wetland was selected as potentially the most cost effective alternative. The plants in a wetland system are an essential component in the process, but contrary to popular belief they do not serve a significant role via plant uptake of the pollutants. The major pathway for pollutant removal is biological via the microorganisms which live and grow in the system. The physical presence of the plants is essential since they provide host surfaces for the attached growth organisms and in the FWS case the canopy of leaves shades the surface and prevents detrimental algae growth. The plant roots are also thought to be a partial source of oxygen in the SF case. The performance of a wetland system is a complex relationship based on a number of ir.terrelated factors. It is considered to be an attached growth biological reactor, similar to trickling filters and RBC's in describing the function of the microorganisms in the system. These reactions are dependent on detention time or surface loading rates, and on the temperature in the system. The wetland, however, differs from conventional short detention time systems in that the flow rate, and detention time or loading, is influenced by evapotranspiration, precipitation and seepage and these factors must be taken into account in a final design. Another unique aspect of wetland performance is the result of the plant and biological community living in the wetland. As a result, it is not possible to achieve complete removal of BOD, TSS, or the nitrogen species. The decomposition of these natural system components will leave a residual BOD, TSS and nitrogen in the final effluent. The residual concentrations are in the range of 5 to 7 mg/L for BOD and TSS, and < 1 for nitrogen. The factors discussed in the previous paragraphs will be considered during the final design. For this preliminary conceptual design it is assumed that the flow rate will remain constant at 7570 m3/d and the temperature of the water in the wetland will remain at 16°C. A wetland system can be optimized for treatment or for habitat value but not for both factors in the same area. Optimization for treatment generally requires dense vegetation and relatively shallow water « 0.8 m). Optimization for habitat value generally requires a diversity of plant species, deep open water zones, and nesting islands for birds. However, there is some habitat value in a wetland designed for treatment and some treatment value in a habitat wetland. The preferred approach in wetland design is to satisfy the treatment requirements first and then provide more suitable habitat at the end of the system if the project permits such an approach. This is the approach taken for the proposed Hornsby Bend wetland.

BOD REMOVAL Both BOD and TSS are critical parameters for the reuse intentions at Hornsby Bend. Experience indicates that of these two BOD will be the limiting design factor. In the general case with typical wastewaters a system sized to remove BOD to expected levels will also satisfactorily remove TSS. The approach used for Hornsby Bend is to size the wetland for BOD removal and then calculate the removal capability of that wetland size for TSS and nitrogen removal.

Sludge management and wetland habitat

217

There is ~ot a universal consensus on design procedures for these wetlan d systems. One school of thought bases desIgn on first order plug flow reactor kinetics with detention time and temperature as controlling parameters. This approach is proposed by Reed et ai. (1995). The second approach also uses first order plu~ flow reactor kinetics but bases design on surface loading. This procedure can be found in Kadlec and KnIght (l~95). The first approach is used for the basic calculation in this paper and then the results ch~cked wIth the second model. It is reasonable to expect that either model should give similar results. Usmg the model developed by Reed (1995), BOD removal in a FWS wetlan d can be characterized by: Ce / Co = exp (-KTt)

(1)

where, Ce = effluent BOD, mg/L Co = influent BOD, mg/L KT = temperature dependent rate constant, d- 1 t = hydraulic residence time in wetland, d (T - 20°)

KT

K20

= K 20 (l. 06)

(2)

= 0.678 d- 1 for FWS wetlands t = LWdn / Q

(3)

L = length of wetland bed, m W = width of wetland bed, m d = average depth of water in wetland, m n = effective porosity of wetland (plants and detritus occupy some space) n = 0.65 to 0.75 depending on plant density and maturity. Q = average flow in wetland, m3/d Equations 1 and 3 can be combined and rearranged to determine the required wetland area to achieve a particular effluent level: A = Q(ln Co / Cc) s Krd n. where,

As

(4)

= surface area (bottom) of wetland, m2

Design models for nitrogen removal are similar in form to Equati ons 1-4 but the rate constants are different. In using these equations it must be remembered that a residu al BOD in the range of 5 - 7 mg/L will always be present in the wetland effluent so the design model should not be used for lower values. The Kadlec/Knight model is similar in form to equations 1 and 4 except that the residual BOD is included as a factor in the equation and the rate constant is a different form. At 15.5°C the rate constant for equation 4 would be 0.522 d-1. The wetland area required to achieve an effluent concentration of 7 mg/L would be 10 ha with a 0.46 m water depth. The Kadlec/Knigh t model would predict an effluent concentration of 6 mg/L for this size wetland indicating that the two models will yield similar results. Figure 1 illustrates this comparison for the Hornsby Bend conditions.

Suspended Solids Removal The Reed model for suspended solids removal is written in terms of hydrau lic loading on the system:

S. REED

21X

el

al.

DBTBNTlOJI TDD ~ 0

4

J

1

U

S

,

8

8

•

1

I

55

} 50; 45

ICl

i

i

B

0

40

0

JO

~~o

J5 :l~

0 ~

15 • 20 15 10 5 0

1

0

~

.

ft

ft

6

a

C)

•

_8 11

10

W.BTLAND &IlBA 0

IlBBD MODEL

~

14

a

.. .

U

• 10

~..,.)

EADLBC MODBI.

Figure 1. BOD Removal in Hornsby Bend Wetland Ce

= Co [0.1058 + O.OOll(HLR)]

where,

(5)

Ce = effluent suspended solids, mg/L Co = influent suspended solids, mg/L HLR = hydraulic loading rate (annual average), cm/d

The annual average hydraulic loading rate on the 8.28 ha wetland would be 33 m/yr or 9 cm/d. The Reed model would predict an effluent suspended solids of 15 mg/L which is less than the required 15 mg/L. The Kadlec/Knight model is similar in form to equation 1, with the right side of the equation written in terms of a rate constant divided by the annual hydraulic loading. Their model would predict < 10 mg/L for suspended solids.

Nitrogen Removal The Reed model for ammonia removal is similar in form to Equation 1, with a specific temperature dependent rate constant for ammonia. It would predict an effluent ammonia of 6.8 mg/L. The Kadlec/Knight model predicts a total 0f 8.8 mg/L for ammonia and nitrate (CH2MHill, 1995) combined. That prediction of significant nitrate in the effluent from this wetland does not conform with experience elsewhere. In the general case most of the depth in these wetland systems tends to be anoxic or anaerobic. It is therefore difficult for nitrate nitrogen to survive in significant amounts during passage through the wetland. Any nitrate produced in the wetland also tends to be rapidly denitrified. Assuming the ammonia in the wetland influent represents about 60 percent of the TKN nitrogen, the TKN nitrogen concentration entering the wetland would be about 17 mg/L. Any nitrate in that influent should be readily denitrified in the wetland as discussed above. On that basis, the Reed model would predict about 5 mg/L TN in the effluent and the Kadlec/Knight model about 7 mg/L for the same sized wetland.

DISCUSSION The 10 ha wetland satisfies the requirements for BOD, TSS and nitrogen removal. This is less than one half the total area (24.4 ha) of the lagoon which has been designated for conversion to a wetland. The final size of the treatment portion of this wetland will be determined during final design. It is suggested at this point that about two thirds of the area or 16 ha be developed for optimum treatment, and the remainder of

Sludge management and wetland habitat

219

the 24 ha be optimized for habitat values with deep water zones, greater plant diversity, nesting islands, etc. Public access to this portion of the facility is strongly recommended. The City of Austin, Texas has already established a Center for Environmental Research at the Hornsby Bend facility so the wetland treatment site can be incorporated in their public information program and access provided for bird watching and other environmental activities. These results indicate that the existing greenhouse facility is not required for treatment of the sludge filtrate for reuse purposes. However, it is more convenient to route the wetland effluent through the greenhouse basins since the piping already exists. The three basins in the greenhouse have a bottom area of about 1 ha. Consideration should be given to changing the water levels in these basins and planting them with emergent wetland plants to further polish the effluent prior to reuse. The proposed water level would be at the soil surface at the inlet end of the basins and about 0.6 m at the effluent end. This additional wetland area would provide significant additional treatment, with minimal operation and maintenance requirements.

Costs The costs for modification and conversion to a wetland of the existing 24.4 ha lagoon cell are estimated at $390,000 (US$) (CH2MHill, 1995), And the cost for annual maintenance at $11,700 per year. These construction costs may increase slightly if the last third of the wetland is optimized for habitat values. Converting the greenhouse basins to wetland channels might add another $15,000 to the construction costs. The wetland component represents less than five percent of the total life cycle costs of the propose filtrate treatment system.

CONCLUSIONS The use of a wetland system as a treatment component for water reuse at the Hornsby Bend facility provides a low cost method for treatment with very low maintenance costs. The concept will also provide significant new habit for birds and wildlife, and public access to this portion of the facility is recommended. Two alternative design approaches for wetland systems were utilized and provide reasonably close predictions of expected effluen facility is recommended. Two alternate design approaches for wetland systems were utilized and provide reasonably close predictions of expected effluent quality .

REFERENCES CH2MHill, (1995). Preliminary Engineering and Environmental Considerations Report. Hornsby Bend Sludge Management Facility Improvements, Volume II of II, CH2MHill, Austin, Texas. Kadlec,RH. and Knight, R.L. (1995). Wetlands for Wastewater Treatment, Lewis Publishers, Chelsea, MI, USA, (in press). Knight, RL., Kadlec, R.H., and Reed, S.C. (1993). Database-North American Wetlands for Water Quality Treatment. US. EPA, Cincinnati, OH Reed, S.C., Crites, R.W., and Middlebrooks, E.1.(1995). Natural Systems for Waste Management and Treatment. McGraw Hill Book Co., New York, NY U.S. EPA, (1988). Design Manual - Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment. US. EPA 625/1-88/022. Cincinnati, Ohio.