Scholarly article on topic 'Vertical Subsurface Wetlands for Wastewater Purification'

Vertical Subsurface Wetlands for Wastewater Purification Academic research paper on "Earth and related environmental sciences"

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{"Vertical subsurface wetlands" / "Wastewater depuration"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — M.M. Pérez Villar, E.R. Domínguez, F. Tack, J.M. Hernández Ruiz, R. Sánchez Morales, et al.

Abstract In this paper, firstly, was carry out the design of a vertical subsurface flow wetland, this treatment system was designed to be placed after the already existing treatments in the company of weld rails (SOLCAR) in Placetas Cuba, the existent treatment in this company consists of septic moat and a settlement. These treatments do not reduce the pollutants below the limits of download, these wastewaters are discharged to a basin of national importance because water supplies to the population. The characterization of the wastewater was realized according to Standard Methods for the Examination of Water and Wastewater and show high levels of nutrients and organic matter, as well as, low levels of dissolved oxygen, having anoxic characteristics the wastewater effluent from the settlement. Is proposed a vertical subsurface wetland by characteristics of wastewater with low levels of dissolved oxygen and intermittent feeding (twice a week), the calculation of the necessary area for the wetland was carried out considering the kinetic behaviour of these systems like of first order for the removal of the BOD. A vertical subsurface wetland with a superficial area of approximately 20 m2 with 0.8 m of height and as substrate the red ferralitic soil that presents high contain of iron and aluminium, which favours the phosphorus removal, planted with Scirpus Alternifolios. After of the wetland construction was realized an evaluation of the efficiency in the purification of the main polluting agents, results demonstrated the use of vertical subsurface flow wetland as an appropriate treatment to reduce pollutants in wastewater, having been obtained a significant increase of dissolved oxygen (5mg/L), the reduction of the nutrients in the effluent wastewater (around 1.5mg of total phosphorus per litre and 2mg of total nitrogen per litre) and the level of BOD (6mg/L) and COD (15mg/L) also were reduced. According to these results, it can be concluded that the wetland system utilized in this research could be a suitable solution for wastewater purification.

Academic research paper on topic "Vertical Subsurface Wetlands for Wastewater Purification"

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Engineering

Procedía

ELSEVIER

Procedía Engineering 42 (2012) 2145 - 2153

www.elsevier.com/locate/procedia

20th International Congress of Chemical and Process Engineering CHISA 2012 25 - 29 August 2012, Prague, Czech Republic

Vertical subsurface wetlands for wastewater purification

M. M. Pérez Villara a*9 E. R. Domíngueza? F. Tack\ J. M. Hernández Ruiza, R.

Sánchez Moralesa9 L. E. Arteagaa

aFaculty ofChemistry andPharmacy, Central University "Marta Abreu" ofLas Villas, Road to CamajuaniKm. 5 V2, Cuba

In this paper, firstly, was carry out the design of a vertical subsurface flow wetland, this treatment system was designed to be placed after the already existing treatments in the company of weld rails (SOLCAR) in Placetas Cuba, the existent treatment in this company consists of septic moat and a settlement. These treatments do not reduce the pollutants below the limits of download, these wastewaters are discharged to a basin of national importance because water supplies to the population. The characterization of the wastewater was realized according to Standard Methods for the Examination of Water and Wastewater and show high levels of nutrients and organic matter, as well as, low levels of dissolved oxygen, having anoxic characteristics the wastewater effluent from the settlement. Is proposed a vertical subsurface wetland by characteristics of wastewater with low levels of dissolved oxygen and intermittent feeding (twice a week), the calculation of the necessary area for the wetland was carried out considering the kinetic behaviour of these systems like of first order for the removal of the BOD. A vertical subsurface wetland with a superficial area of approximately 20 m2 with 0.8 m of height and as substrate the red ferralitic soil that presents high contain of iron and aluminium, which favours the phosphorus removal, planted with Scirpus Alternifolios. After of the wetland construction was realized an evaluation of the efficiency in the purification of the main polluting agents, results demonstrated the use of vertical subsurface flow wetland as an appropriate treatment to reduce pollutants in wastewater, having been obtained a significant increase of dissolved oxygen (5 mg/ L), the reduction of the nutrients in the effluent wastewater (around 1.5 mg of total phosphorus per litre and 2 mg of total nitrogen per litre) and the level of BOD (6 mg/L) and COD (15 mg/L) also were reduced. According to these results, it can be concluded that the wetland system utilized in this research could be a suitable solution for wastewater purification.

© 2012 Published by Elsevier Ltd. Selection under responsibility of the Congress Scientific Committee (Petr Kluson)

Keywords: Vertical subsurface wetlands; wastewater depuration

a* Corresponding author. Tel.: + 0053-422-81510; fax: +0053- 422-81608. E-mail address: mairapv@uclv.edu.cu

Faculty of Bioscience Engineering, GhentUniversity, Coupure Links 653, B9000, Belgium

Abstract

1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.07.592

1. Introduction

As alternative to the conventional techniques of purification have been developed a series of natural systems which take improvement the purification processes that occur in a spontaneous way in the nature, in them the purification is gotten through the natural physical, chemical and biological processes, developed in a system plant-soil-water[l].

Constructed wetlands are including in the natural methods of wastewater depuration. They are designed to take advantage of many of the processes that occur in natural wetlands, but do so within a more environment controlled [2-5].

Wastewater treatment is achieved in a constructed wetland through an integrated combination of biological, physical and chemical interactions among plants, substrate and microorganisms. Due to the minimum requirement for their operation and management, constructed wetlands are promoted as low-cost technology for the wastewater treatment [6-9].

Constructed wetlands are generally classified into two main types: surface flow and subsurface flow constructed wetlands that are again subdivided into horizontal flow and vertical flow constructed wetlands, depending on the direction of the water flow through the porous medium (soil or gravel) The advantages cited for subsurface flow wetlands are greater cold tolerance, minimization of pest and odor problems, and, possibly, greater assimilation potential per unit of land area than in surface flow systems. The wetlands with horizontal flow works permanently flooded these systems to have the oxygen as limiting, because plants are not capable of supplying oxygen to the speed required by the organic loading and nitrification tend to occur at very low levels [10]. The wetlands with vertical flow are designed with intermittent operation. The intermittence flow confers to properties different to the vertical and horizontal systems [11].

In consideration of their good efficiency and relatively small land requirement, vertical flow reed beds have been increasingly used as a popular alternative for the treatment of a wide range of wastewaters [1213]. The modality of feeding of the vertical system allows a bigger oxygenation of the liquid one and then a bigger capacity of degradation of the organic matter being reached removals of 85 and 95% for the COD and BOD respectively and can increase the nitrogen removal in the wetland [2], [14-15].

Although removal performance of main pollutant has been favourable in a vertical wetland, long term studies and increased operational experience indicate that phosphorus removal is variable or inconsistent. These inconsistencies can be attributed to the complexity of phosphorus removal mechanisms. The selection of substrate with high content of minerals is of main importance to remove phosphorus in these systems [16].

The present paper has as purpose designing, constructing and evaluating a vertical subsurface wetland, for wastewater treatment of weld rails industry (SOLCAR) in the central region of Cuba.

Nomenclature

Cm,C0utinfluent and effluentpollutant concentration t hydraulic retention time k kinetic constant y depth n porosity Q flow

2. Material and Methods

2.1 Wetlands Design

First-order degradation represents the basic design equation (equation 1) employed widely in Australia [17], Europe [18-19] and the USA [20-21]. The design of vertical subsurface wetlands was carried out with this equation for BOD removal, Cout ' kt

P.' (1)

and the retention time equation is:

t As y, n (2)

Combining the equation 1 and equation 2 we obtain the equation 3, by which calculate the area of the wetland.

Q In Cin / Cout

A —---(3)

5 TS- sb . sb .. \ 1

2.2 Analyticalmethods

The affluent wastewater was characterized over a one-year period. The following parameters were determined, based on standard methods [22]: pH and electrical conductivity were taken in the field using a Hanna HI9811-5 portable meter. Chemical oxygen demand by Closed Reflux dichromate methods, biochemical oxygen demand was measured by 5-Day Winkler methods and dissolver oxygen by Winkler method. Total suspended solids (TSS) were determined by Total Solids Dried at 103-105 °C. Total Kjeldahl nitrogen and ammonia nitrogen were determined by an automated PRO NITRO II Kjeldahl. Nitrate was measured by nitrate electrode method with pH/ion meter WTW-735. Total phosphorus and nitrite were determined by colorimetric methods using UV Genesys 10.

Redox potential (Eh) was measured at different depth of wetland bed, platinum-tipped electrodes was inserted into the substrate using a pH meter Hydrocheck CD7000.

3. Result and analysis

3.1 Wetlands Designs and Construction 3.1.1 Localization

The wastewater treatment in weld rails industry only has a primary treatment that consists in a septic moat and a settlement. This treatment system not reduces the pollution according to regulatory level in Cuba NC27 [23] and the effluent wastewater present anoxic characteristic.

Figure 1 show this primary treatment system in SOLCAR.

The combination of the existent treatment in SOLCAR with the vertical subsurface wetland, can allow compliance the quality requirements for wastewater discharge. It is propose a vertical subsurface wetland due to this treatment system operated with loads superiors that the horizontal ones and produce more oxygenated effluents.

Fig. 1. Primarytreatment in SOLCAR

Wastewater presents the following characteristics:

> Small flow (9 m3/day)

> Domestic characteristic.

> Preliminary treatment to remove solids, allowing for better functioning of the wetland.

Table 1 presented the media value and the efficiency removal of the main pollution of the influent wastewater and the regulatory level.

Table 1. Influentwastewater characterization

Parameter Regulatory level Influent

PH(U) 6,5-8,5 7,25 ± 0,36

Electrical conductivity (^S/cm) 1400 690 ± 106

COD (mg/1) 70 97,94 ± 16,77

BOD5 (mg/1) 30 42,17 ± 6,10

Dissolved oxygen (mg/1) - 1,07 ± 0,42

Total phosphorus (mg/1) 2 7,11±0,85

Kjeldahl nitrogen (mg/1) 5 13,98 ± 3,42

NH4 Nitrogen (mg/1) - 2,59 ± 1,81

N02 Nitrogen (mg/1) - 0,03 ± 0,02

N03 Nitrogen (mg/1) - 4,98 ± 2,69

Total suspended solid (mg/1) - 19,75 ± 4,35

3.1.2 Design parameter

Substrate: Red ferralitic soil, placed over a gravel layer, used to evade the obstruction in the perforated discharge pipe. This substrate presents high contain of Fe and Al, which favours the P removal [24-25]

Plant: Cyperus Alternifolius, this plant is with great abundant in the area and is reported with good results the purification in wetlands [26-27] Waterproof material: Clay compacted Dimensions:

The superficial area was calculated using the equation 3 and the following dates: Q: 9 m3/d Cm: 42 mg/1 Co* 15 mg/1

k: 1.104 d-1 according to Reed [20] y: 0,8 m n: 0.54 A= 20 m2

Considering the relation length- width ration of 3:1 will be obtained the wetland with approximately:

3mof width

7mof length

Entrance and discharge structure: Entrance structure: The distribution pipe network will be constituted by a perforate pipes placed across the width of the wetland fed to the perforated pipes placed along the wetland.

Discharge structure: Perforates pipes in the bottom of the wetland.

Also will be placed vertical aeration pipes to oxygenate the deeper layers of the bed.

3.1.3 Construction

For the wetland construction, was prepared the land, and waterproofed with clay. Then, is made the filling and conditioning where is placed the outlet pipe and the air pipes and a gravel layer to prevent obstruction in the outlet pipe. Finally was placed the substrate and was planted the Cyperus .Alternifolius (Fig. 2).

Fig. 2. (a)Wetland Waterproofing; (b) Wetland Filling and conditioning; c) Planted wetland

3.2 Wetland performance

It was obtained a significant reduction of pollutants in the effluent wastewater after 12 weeks of operation of the vertical subsurface wetland. The values obtained for all parameters are according to the Cuban regulations for the disposal. Notable is the significant increase in dissolved oxygen in the wastewater effluent (5.45 mg/1) with respect to the influent wastewater (1.07 mg/1). The total phosphorus removal efficiency in the vertical subsurface wetland (76%) was greater than the phosphorus efficiencies reported typically for gravel substrate (20-30%) [28].

Table 2. Effluent wastewater characterization

Parameter Regulatory level Effluent

PH(U) 6.5-8,5 7,04 ± 0,02 -

Electrical conductivity (^S/cm) 1400 677 ± 18 -

COD (mg/1) 70 10,04 ± 3,85 89,8

BOD5 (mg/1) 30 6,39 ± 3,5 84,9

Dissolved oxygen (mg/1) - 5,45 ± 0,21 -

Total phosphorus (mg/1) 2 1,67 ± 0,23 76,5

Kjeldahl nitrogen (mg/1) 5 2,43 ± 0,22 82,7

NH4 Nitrogen (mg/1) - 1,53 ± 0,22 82,2

N02 Nitrogen (mg/1) - 0,03 ± 0,001 -

N03 Nitrogen (mg/1) - 8,02 ± 0,27 -

Total suspended solid (mg/1) - 0,38 ± 0,10 98.1

Fig 3, show the behaviour of nutrient and organic matter and the Fig 4, show the behavior of dissolver oxygen and nitrate by twelve first week of wetlands. A stabilization of the nutrient values is observed from the sixth week of operation. The COD and dissolver oxygen values present a stable performance after the fourth week of operation. It is observed an increase of nitrate concentration in the effluent wastewater, corroborating the oxidation condition in the wetland bed.

The redox potential change in the wetland bed, obtaining values around 650 mv in the most superficial areas and near to the roots of the plants and of around -100 mv in the deepest areas in the channel, what can favor the main removal mechanism proposed for the nitrogen that is that of nitrificacion-denitrificacion.

■regulatory ]evd phosphorus -»-rigiilatory levd nitrogen —t—COD -B-reaulatop. ]evd

Fig. 3. (a) Nutrient behaviour; (b) COD behaviour

Fig. 4. (a) Dissolver oxygen behavior; (b). Nitrate behavior.

Conclusions

The area necessary, for effective BOD reduction in wastewater treatment, of the vertical subsurface wetland is the 20 m2.

The vertical subsurface wetland after a primary treatment in SOLCAR reduces the value of main pollutant in the wastewater and it presents a stable behavior in the reduction of the main pollutants after the first six weeks of operation.

The aerobic condition in the superficial wetland bed and anaerobic condition in the bottom allow the nitrogen removal in the wetland.

The high phosphorus removal efficiency in the vertical subsurface wetland with red ferralitic soil was obtained.

Acknowledgements

This work has been financially supported by Project 7, Environmental Education and Development of Clean Technologies of VLIR Program.

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