ELSEVIER Desalination 106 (1996) 24 l-246

DESALINATION

Recycling nutrients from municipal wastewater

Brace H. Boydena*, Abdellah A. Rababahb

aAquatec-Maxcon, linit 4, 6 ByJeId Street, North Ryde (Sydney), Australia 2113

Fax: +6/ (2) 888-3044

bCentre for Wastewater Treatment, School of Civil Engineering, University of New South Wales,

Sydney, Australia 2052

Received 3 May 1995; Accepted 12 September 1995

Abstract

A commercial hydroponic system is being adapted for studying the potential recycling of nutrients from settled primary domestic wastewater to produce value-added crops. The system is 3 m long, with small gullies used for easily manageable plants like lettuce and capsicum, and large gullies for large plants like corn and tomato. Primary treated effluent is being used to irrigate lettuce in one series, and a commercial nutrients solution is being used to irrigate the same type of lettuce in another series as a control. The crops grown in these systems have shown a remarkable ability to remove nitrogen and phosphorous from settled primary sewage. Studies are continuing to improve hydroponic design and to accommodate other crops. Early results have shown that lettuce will remove over 77% of the phosphorous and 80% of the nitrogen. The crops irrigated with sewage without disinfection appear healthy compared to the control using commercial nutrients but yield plants which are up to 50% less mass. At the end of the cropping period, the plants were harvested to quantify total nutrient uptake and to test for heavy metals in the edible portion of the vegetables.

Keywords: Wastewater treatment; BNR; Reuse; Hydroponics; Macrophytes

1, Introduction

The real solution for polluted waterways and water shortages is wastewater reuse. Wastewater irrigation can supply almost all nitrogen and most of the phosphorus and other micronutrients re- quired by many crops.

Reused wastewater for irrigation is considered to be a major and vital resource in many parts of the world [l-6]. Regions such as North and

South Africa, the Middle East, southern Europe, southwestern US, Mexico, South America, and parts of central and east Asia. Mostly this is due to the combination of the following conditions: (a) severe water shortages, (b) a pollution threat to diminishing water resources, (c) a highly con- centrated urban population, (d) intensive agricul- tural irrigation, and (e) high environmental awareness of the public and the general accep- tance of the public for the need of recycling and

*Corresponding author. reuse.

@Ill-9164/96/S] 5.00 Copyright 0 1996 Elsevier Science 3.V. All rights reserved. PII SO01 l-9164(96)001 14-2

242 B.H. Boyden, A.A. Rababah / Desalination 106 (1996) 241-246

Australia is the driest inhabited continent on earth, yet it only reuses 5% of its wastewater, mostly for the irrigation of golf courses, as wash- down water for growth of forage grasses or as industrial cooling water [7]. Moreover, excess nutrients from sewage treatment plants and from fertilizer wash-off end up in rivers to contribute to eutrophication and the growth of blue-green algae. Recent draught crises in New South Wales and Queensland are causing Australia economic loss, land degradation, and even making Australia unable to meet its commitment to provide over- seas countries with already signed contracts, as is the case to provide Japan with wheat. Wastewater reuse projects are essential to ameliorating this and future draught crises.

A pilot-scale experimental system was in- stalled at an experimental site belonging to the School of Civil Engineering, University of New South Wales, Australia, to study the possibility of adapting a commercial hydroponics system for (1) nutrients removal from sewage and (2) grow- ing vegetables fit for human consumption or as animal fodder.

This paper presents the initial phase of a 3 y project aimed at hydroponic design enhancement to accommodate wastewater usage, microbiologi- cal studies regarding contamination of the plants by water-borne pathogens as well as the quanti- fication of any concentration of heavy metals within the plants themselves. A commercial hy- droponics system utilizing a commercial nutrient solution is run in parallel, with the system using wastewater, as a control. The removal of nutri- ents and quantification of plant growth are cov- ered in this paper. The quality of the products will be assessed at a later stage.

2. Materials and methods

2.1. Wastewater plot

Primary settled municipal effluent is regularly shipped from Liverpool sewage treatment plant (western Sydney). The wastewater is pumped from the primary settling tank into a mobile

fibreglass tank and transported to the experimen- tal pilot plant. This tank is fixed onto a trailer, along with a petrol driven pump. The mobile system was specially designed and manufactured for the project. From the mobile fibreglass tank, the wastewater is transferred into a 1 m3 plastic storage tank from which the wastewater is pumped around a close-loop hydroponic-type system. After a significant portion of the nitrogen and phosphorous is removed, fresh effluent is added to the system. Fig. 1A shows the various elements of the pilot plant.

2.2. Control plot

Parallel with the wastewater plot (Fig, lB), a commercial hydroponics system is operated which uses a commercial nutrients solution dis- solved in ordinary tap water. This solution is pumped from a 60 1 tank to the plants in a similar hydroponic system to those of the wastewater plot. The used solution flows back to the 60 1 plastic tank by gravity.

2.3. Equipment for wastewater plot

. Storage tank - A cylindrical 1 m3 plastic tank was selected to store the sewage after being shipped from the treatment plant. The volume of the tank was calculated to suit the required flow

l Transmission pipeline - A light 3/S”-plastic pipe was selected to transmit the sewage from the storage tank to the plants. This type of pipe has been successfully used by commercial growers throughout Sydney for over 14 y. The pipe per- formed well throughout the study and was found to be easily handled with minimum head loss and deposits around its inner surface. l Pumps - a little giant submersible pump (from Trans World Traders Pty Ltd, Sydney) was used to pump from the storage tank to the plants. This pump can deliver up to 15 l/min at a 5 m head loss, This pump is larger than the pump required by the control plot due to the require- ment of higher flow rates needed because of

B.H. Boyden, A.A. Rababah / Desalination 106 (1996) 241-246 243

Fig. 1A. Hydroponic system for wastewater.

/

/ nutrient-fresh water tank kits with gullys of 56 plants capacity

Fig. 1B. Commercial hydroponics system.

smaller quantities of nutrients. A 3/S”-plastic valve was installed after each pump to control the flow rate. l Growing media - Perlite/vermiculite were used as support media. A ratio of two parts per- lite to one part vermiculite was the prevailing mixture used. The vermiculite keeps the media moist while the perlite helps in aeration. l Growing lights -Lighting systems were used to control light dosage to both plots. Six 400-W halide lights with a light intensity of 450 pW/m* and a wavelength around 500 nm were installed overhead the hydroponics plots. A 400 W light suits an area of 1 m2, which accommodates 10 plants for this project. The lights were kept 300 mm over the plants for up to 16 h/d. . Plants - Preliminary criteria for the selection of plants to be tested with wastewater included: (1) preliminary selection of those plants grown commonly with hydroponics system to minimize initial system modification; (2) concentration on those species requiring large nitrogen and phos- phorous inputs; (3) plants must be able to tolerate the wastewater’s physical and chemical charac- teristics; (4) plants must well grow using growing lights with short maturity cycles (e.g., S-10 weeks); and (5) plants should have a commercial

value. After some consultation, the project was initiated with Mignonette Green Lettuce. Capsi- cum was selected as a second possibility and flowering crops will be tested in the future, l Wastewater - Primary settled wastewater was used in this study before removal of nitrogen and phosphorous. Initially the project used prima- ry settled wastewater from Malabar STP. How- ever, after CAS was installed, phosphorous levels were too low. A good replacement in terms of proximity and nutrient concentrations was Liver- pool STP. Liverpool sewage treatment plant primary effluent is settled without chemical addi- tion. Effluent is shipped weekly from the site. Every shipment of wastewater is analyzed in the laboratory before being applied to the roots of the plants. The average characteristics of the waste- water are listed in Table 1.

2.4. Equipment for control plot

The control plot is in principle the same hy- droponic system used by hobby growers. It con- sists of a 60 1 tank for nutrients solution storage, a small pump, and the gullies where the plants are grown. The pump is a Rena C-40 submersible pump, 10 1 @ 1.4 m head. This pump is of the standard and quality required for the use in hy- droponics. The pump is robust, reliable and is able to tolerate nutrient solution salt levels with- out causing contamination of nutrients or rapid deterioration of the pump itself. A plastic by-bass

valve is used to control the quantity of water being pumped to the plants.

Commercial nutrients were purchased from Accent Hydroponics. Their nutrients starter “cul- ture s” is packed in two separate A&B bags to keep sulphate and calcium concentrates separate to prevent precipitation. Mixture A (500 g) was dissolved in 2 1 of water (solution A) and 500 g of mixture B were dissolved in 2 1 of water (solu- tion B). Two ml of solution A and 2 ml of solu- tion B were added to every like of irrigation

water. The plants were irrigated at 6 l/min with the final diluted nutrients solution in the control plot.

244 B.H. Boyden, A.A. Rababah /Desalination 106 (1996) 241-246

3. Results and discussion Table 1

In this preliminary study, it was planned to study (1) the efficiency of nitrogen and phospho- rous removal from the wastewater, (2) the weight and health of the plants in each plot, and (3) the concentration of heavy metals in the plants from each plot. Results from the first two objectives are reported herein along with the BOD, COD, and SS analyses of the wastewater.

Wastewater characteristics before and after being used to irrigate the plants

Parameter Initial concen- Finat concen- % tration and Stan- tration and Stan- removal dard deviation, dard deviation, mg/I mg/l

Total N 64 13 80 Total P 6.2 f 3 4.2 + 2 77 Nitrate 18.2 f 4 36 k6 NA BOD 195 f 13 25 +2 87 COD 366 f 16 53 *5 86 ss 150 + 10 2.4 k 1.7 99

3.1. Wastewater results

The wastewater was analyzed in the laborato- ry before and after being pumped into the hydro- ponic section of the pilot plant. The average results of the analyses are presented in Table 1. Wastewater samples reflect the effects of 1 IO cycles. Plants were irrigated with the wastewater at 9 I/min in the sewage plot, and the average time between wastewater additions was f week.

Table 1 shows good nutrient removal over the 7 d time period. More than 80% of nitrogen was removed from the wastewater by applying it to the roots of the plant, and 77% of total phospho- rous was also removed. Table 1 shows that 70% BOD removal and 99% suspended solids removal were also achieved. Fig. 2 is a graphic represen- tation of the obtained results.

Fig. 2. Percentage removal of organics from sewage in lettuce.

Table 2 Characteristics of nutrients solution, mg/l

Parameter

Total N Total P

Cont. before

208 62

Cont. after

30 8

% removal

86 88

3.2. Nutrients solution results

Laboratory analyses were also carried out on the solution in the control plot. The results of a typical analysis are presented in Table 2, Data in column three represent results from analyzing nutrient solution samples after 1008 cycles based on a flow rate of 6 l/min and the average time between solution additions of 1 week.

From Table 2, it is clear that plants grown in a hydroponic system absorbed most of the avail- able nutrients within a short period of time. Based on the above mentioned flow rates and data presented in Tables 1 and 2, the availability of nitrogen and phosphorous, i.e., what the roots are exposed to for each plant per minute, at the (1) start of a fresh batch of wastewater or nutri-

ent solution, at the (2) end of cycle prior to addi- tion of new nutrient sources and of the (3) aver- age during the cycle is presented for comparison in Table 3.

B.H. Boyden, A.A. Rababah / Desalination 106 (1996) 241-246 245

Table 3 Comparison study of plants irrigated with wastewater and those irrigated with nutrients solution (mgl min.plant)

Wastewater

Parameter Starting value

Ending value

Average removed

Total N 19.9 4 15.4 Fig. 3. Growth rate of plants with time.

Total P 1.9 1.3 0.6

Control growth, possibly due to inadequate potassium or trace elements.

Total N 43 6.2 36.8

Total P 12.8 1.7 11.1

3.3. Lettuce growth

One of the main objectives of the study is to assess the possibility of using the commercial hydroponics to grow high-cash-return crops with primary treated wastewater. Whether disinfection is required to use this wastewater will be treated in future studies.

To ascertain growing rates, a number of mature lettuce plants were periodically harvested from both plots for mass assessment as seen in Fig. 3. The plants of the control plot were larger than those of the sewage plot. Plants in the con- trol plot followed a three-phase growth cycle similar to bacterial growth, viz a lag phase, a phase of exponential growth and the transition into a stationary phase at maturity. Conversely, the plot using wastewater did not follow this three-phase behaviour but rather a simple expo- nential growth as modelled by Eq. (1).

me> m (t=70)

= exp(O.ll%r-8.0035)(,)

I= -0.9935

where m(t) is the average dry plant mass (g) at a time t (d) and m(70) was the final average mass at t=70 d. From Fig. 3, the lettuce using the wastewater apparently did not achieve mature

4. Conclusions and recommendations

The first stage of the 3 y project has shown that it is possible to raise crops successfully with municipal effluent. It is possible to irrigate vege- tables with sewage in a hydroponic system where large quantities of nutrients are efficiently removed in a short period of time. Additional nutrient such as potassium may have to be added to yield a truly commercial crop.

The plants of the sewage plot appeared healthy but were stunted (in terms of mass) as compared to the control group. Growth rates between days 50-70 of both curves in Fig. 3 show that plants in the wastewater system grew at a rate of 375 mg/d, while the growth rate of the plants irrigated by nutrients solution was 705 mg/d. The commercially grown lettuce fol- lowed a three-phase growth curve, while the lettuce grown in wastewater could be modelled with a single exponential curve.

The results have also shown surprisingly good removal of BOD, COD, and SS. More research is needed to concentrate on health aspects of the produced products. Such research will include plant analyses for heavy metals, microbial (virus and bacteria), and nutrients level. Future research is also required to improve the existing hydro- ponics designs to accommodate wastewater for a wider variety of plants,

246 B.H. Boyden, A.A. Rababah / Desalination 106 (1996) 241-246

Acknowledgment

The authors would like to thank Accent Hy- droponics pty Ltd. who donated the equipment for this project.

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