Start-Up of a Novel Reed Bed System for Agricultural Wastewater Treatment

Start-up of a novel reed bed system for

agricultural wastewater treatment

Y. Q. Zhao, R. Connolly, G. Sun & S. J. Allen

School of Chemical Engineering, Queen's University Belfast, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, UK.

Abstract

This paper describes the experimental investigation of a five-staged tidal flow reed bed system for the treatment of an agricultural wastewater from a pig farm. In particular, study focuses on the start-up of the reed bed treatment process. Initial investigation was concerned with batch operation of the system with addition of an artificial nutriment, commercial baby bio, to promote growth of the reeds; emphasis was placed on how to establish a healthy microorganism population within the bed matrix, i.e. period in which plants and microbes adjust to the hydrological conditions of the new living environment. Thereafter, diluted pig slurry was introduced into the reed bed system that adopted a novel operation of 'tidal flow'. Strength of the influent was progressively increased. Under a continuous operational condition, the reed bed system was successfully started up. Stable removal rates of 82.3% and 83.6% were established for BOD5 and COD, respectively, during the continuous operation.

1 Introduction

Constructed wetlands are known as reed bed systems when common reeds,

Phragmites australis, are planted as the main macrophytes. Reed beds offer a low-cost and environmental friendly treatment technique in a category of what is increasingly termed as 'green' technology. This technique works on the principle that reeds have the ability to transfer oxygen to their rhizomatous roots, stimulating bacterial growth in bed matrices and breaking down pollutants in the root zone [l].

304 Water Pollution \//I: Modclling, Mca~uringand Prediction

In recent years a new operating method for reed beds, alternately filling and draining matrices with wastewaters, has been proposed [2-41. Ths new operation created a novel system, namely 'tidal flow reed beds'. Fully submerging bed matrices during the filling process provides maximum media-wastewater contact and avoids the problem of poor wastewater distribution often associated with conventional reed bed systems. Subsequent draining process enables air to be drawn from the atmosphere into the bed matrices, thereby enhancing aeration and stimulating aerobic biological processes to decompose organic pollutants and amrnoniacal-nitrogen. It is believed that successful performance of reed beds depends highly on start-up operation. Unfortunately, little information is available regardmg the start-up of any reed bed system [5]. In this study, investigations were made on the start-up behaviour of a gravel-based tidal flow reed bed system that was designed for the purpose of treating high strength agricultural effluents.

2 Materials and methods

The experimental reed bed system consists of five identical beds that were made of Perspex columns of 90 cm in height and 10 cm in diameter. Each bed was filled with 26.4 f 7.2 mm washed round gravel to a depth of 15 cm as supporting layer, followed by a top layer of 4.4 f 1.5 mm washed gravel with a depth of 65 cm. Figure 1 presents a schematic diagram of the reed bed system. A common reed, Phragrnites australis, from a commercial supplier, was planted in the top layer of each bed.

Peristaltic pump

I Feed tank Reed beds

Figure 1 Schematic diagram of the tidal flow reed bed system

The start-up of the system was carried out in two periods, namely batch period and continuous period. Batch experiment period commenced as soon as the reeds

were planted. During the experiment, reed beds were submerged by tap water, and the beds were drained for one day in each week for resting. In order to assist the reeds settling down in the new environment and promote their growth, commercial baby bio was used as a growing nutriment with a dose of 10 drops of baby bio in every 500 rnl of tap water. This solution was introduced by hand to the top of each reed bed every two days for the first ten days and once a week after that time till six weeks. Thereafter, the baby bio solution was used every ten days until 105 days. Continuous experiment begins immediately after 105 days.

In continuous experiment, diluted pig slurry was prepared in a feed tank with COD, BOD5 and NH4-N levels up to 932 mgll, 546 mg/l and 37.7 mgll, respectively. This wastewater was introduced into the reed bed system by peristaltic pumps, as shown in Figure 1. Controlled by timers, six peristaltic pumps generated the 'tides', alternately filling and draining of the bed matrix, which took place in a cycle of every four hours; this gave each reed bed three hours of wastewater-bed matrix contact and one hour of resting. The flow rate of all peristaltic pumps was set as 56 dmin. Samples of influent and effluents

from each stage were collected and analysed for COD, BOD,, m-N, NO,-N, NO3-N, SS, PO4-P and pH. This continuous experiment of tidal flow reed bed treatment was performed with progressively increasing strength of the influent.

3 Results

3.1 Batch experiment period

Main findings during the batch operation period are presented in Figure 2.

4 b Mure reeds gmin beds WastMerms ~ntroduced& contlnuws epa~msstarted~n Babybiovvas 4 b t~dalflowmm a &by bro was dosed every 70 days mk

Figure 2 Sketch of the reed bed behaviour in batch experimental period

Immediately after being planted in the beds the behaviour of reeds in the first several days was worrying as the leaves turned yellow. After about 10 days, it

was found that there was no further worsening of reeds' appearance. From that

306 Water Pollution \//I: Modclling, Mca~uringand Prediction

time, baby bio was added to the system once a week. By dosing the baby bio solution, new growth of reeds in each bed was observed after 18 days. In the following month original stems of the reeds were gradually replaced by some new growing branches. In particular, new roots of the reeds were found in bed matrix after 35 days, which was an indication that the plants had adapted to their new environment. From Day 35 to Day 98 matured reeds grew rapidly in the beds. Deep and massive root development of the reeds was observed on the 98th day. It was noted that some algae grew in bed matrix after 61 days since the reeds were planted. In order to prevent excessive growth of algae, the reed beds were covered with black vinyl. In total, batch operation of the reed bed system lasted for 105 days when reeds were being cultivated and matured. Experiment of continuous wastewater treatment was then started with diluted pig slurry.

3.2 Continuous experiment period

During the continuous experiment wastewater samples were collected from inlet of the system and outlet of each stage. Figure 3 illustrates levels of BOD,, COD, NH4-N and PO4-P of the influent and final effluent. Despite that inlet wastewater concentration was steadily increasing, Figure 3 clearly demonstrated that there were consistent and substantial reductions of COD, BOD5, NH4-N and PO4-P across the reed bed treatment system, as a result of complicated physical, chemical and biological processes in the system [6].Details of the pollutants' removal efficiency were summarised in Table 1. The most notable feature of Table 1 is the increasing removal rates of COD and BODS, fiom 63.2% and 56.6% (COD and BODS respectively) on Day 2 to

83.6% and 82.3% on Day 14. These results were achieved under increasing inlet wastewater concentrations, which corresponded to increasing organic loading onto the reed bed system as the wastewater flow rate remained constant. Table 1 also shows that in general significant reductions of SS and PO4-P were obtained although the data were slightly scattered. Removal of N&-N was in the range of 43-65%. However, there is a trend of decreasing NH4-N removal rate with the introduction of stronger wastewater; the reason could be the accumulation of HN4-N in the reed bed system as the wastewater concentration was rapidly increased, particularly in the fist several days of the continuous experimental period, as shown in Figure 3. Table 2 shows results obtained on Day 9 of the continuous experiment period where a detailed breakdown of treatment results in each stage was presented. Table 2 indicates that the first two stages of the reed bed system played an integral role in the removal of pollutants, mainly COD, BOD5, SS and PO4-P. In particular, the first stage reduced COD and BODS fiom respectively 888 mgll and 546 mgll to 646 mgll and 400 mgll, this being 39.0% of total COD removal and 39.7% of total BOD5 removal in the system. With regard to the SS and PO4-P removals, the first stage contributed to 29.9% and 64.3% of the total removal of SS and PO4-P respectively. It is noted from Table 2 that sustained reduction of NH4-N and NO3-N took place in the 3rd and 4th stages, demonstrating

nitrification and denitrification processes in the system. However, it is noted that there is no obvious change of the wastewater's parameters through the 5th stage, suggesting that only limited pollutant reductions took place here.

Figure 3 Pollutant levels of the influent and effluent of the reed bed system during the continuous experimental period

Table 1. Removal efficiency (%) of pollutants in the reed bed system

308 Water Pollution \//I: Modclling, Mca~uringand Prediction

Table 2. Breakdown of the pollutants across the five-staged reed bed system on the 9th day of continuous operation

4 Discussion

The principal experimental variable under investigation in this study was the start-up of a reed bed system that consists of five stages. The importance of the start-up lies in its close relation with the treatment efficiency of the system. Although there is no defined target of what the start-up operation must achieve, it is believed that at the end of start-up period intense activities of reeds and microorganisms should be established in the reed bed matrix, creating a complex and dynamic biological system in rhrzome areas. This new bio-system will then lead to stable removals of pollutants from wastewater. Due to lack of start-up information for reed bed treatment process, this study aims to investigate the start-up period of a lab-scale reed bed system. Batch operation of the reed bed system aims to collect original formation of the new living environment in which the bed medium, reeds and microbes adjust to the hydrological conditions of the new bed. It is reported that reeds play an essential role in reed bed by stabilising the surface of the beds, providing a suitable environment for microbial growth, opening passageways for wastewater and transporting an amount of oxygen through their root system [5,7]. Therefore, the growth of reeds in the new environment, gravel based reed bed media, has been the focus point of the batch experiment. In this study, it is experienced that the critical period of the reed bed is the first ten days when the leaves of reeds were turning yellow and dry. More importantly, it is baby bio that kept the reeds alive and eventually led to new growth of young reed in 18 days (Figure 2). Accordingly, roots of the reeds started to grow gradually and reached deep ends of the beds afterwards. The growth of reeds both in stem and root could be the evidence of a favourable living environment in the beds. This result is in agreement with claim made by Vanier and Dahab [5] that wastewater should not be introduced until the plants have shown new growth. The current study also reveals the role of artificial nutriment, such as baby bio in promoting the new growth of the reeds. In addition, it must be pointed out that, according to this

study, it is wiser to avoid sunlight to the root area in order to prevent possible over-growth of microorganisms by photosynthesis. Following the batch experiment, continuous experiment was carried out under the tidal flow operation, which was demonstrated as a novel operation option with the advantages of the enhancement of oxygen supply and improved water distribution [4, S]. Continuous operation of reed bed system aims to establish a stable treatment efficiency. Hence, parameters of wastewater and the removal rate will be the main focuses. It was suggested by Lin et al. [9] that, after the initial stabilization period, a gradual introduction of wastewater into reed bed to allow the system to adjust to the new water chemistry is a good way of starting-up. In this study, continuous introduction of wastewater with steadily increasing strength led to high BODs (COD) removal rate of 82.3% (83.6%) within 14 days, as shown in Table 1; such result is encouraging, compared with reports from other systems that periods of 3 months [10], 1 year [l l], 1-2 years [l21 and over 3 years [l31 were taken before the systems reached stable BODS removals. The quickened stabilisation period for the current tidal flow reed bed system may be attributed to the following two reasons:

Firstly, a good new living environment was established during previous batch experiment period that lasted for more than 3 months as shown in Figure 2. After the batch experiment the reed bed system, with extensive development of plant roots, was ready to break down pollutants in wastewater. Although there is no monitory of biological index for living system in batch period, information is available from literature regarding the growth of reeds (stems and roots) to help the formation of living system and improve the purification efficiency. For example, a number of papers reveal that reeds can filter water, control erosion and provide surface for microorganisms, as well as prevent soil clogging

[14,15]. Roots of reeds release O2 into the bed matrix, promoting aerobic conditions for nitrification to occur [16]. The roots may also release exudates that support the growth of microorganisms, and antibiotics that kill pathogens

[6,151. Secondly, tidal flow operation for the reed bed system can be a reasonable factor to achieve high and stable treatment efficiency within a relatively short period. Following current study of the start-up process, Wher studies have been started to investigate the optimal operational method and detailed benefits of the tidal flow reed bed system. Behaviours and abilities of individual stages of the current reed bed system will be examined and reported.

5 Conclusions

Batch operation of current reed bed system is necessary for initial start-up. The aim of the operation lies in the formation of a new living environment, i.e. the bed medium, reeds and microbes adjusting to the hydrological conditions of the new bed. Evaluation of th~soperation is the new growth of reeds both in stem and in root. Artificial nutriment, such as baby bio, is the key factor in this stage. Continuous operation is the final stage of the start-up process. Progressively increasing the strength of wastewater enables the system to reach anticipated

3 10 Water Pollution \//I: Modclling, Mca~uringand Prediction

stable removal efficiency in a relatively short time period. The continuous start- up phase takes several weeks after the previous batch phase has established a proper living environment for the reeds and microorganisms. In addition, tidal flow operation of the reed beds enhances the start-up process and help the system reach the objective of stable pollutant removal efficiency.

Acknowledgement

The authors would like to acknowledge the financial support fiom the EPSRC UK for this work.

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