Cultivation of a new microalga,Micractinium reisseri,in municipal wastewater for nutrient removal,biomass, lipid,and fatty acid production

Coupling of advanced wastewater treatment with microalgae cultivation for low-cost lipid production was demonstrated in this study. The microalgal species Micractinium reisseri and Scenedesmus obliquus were isolated from municipal wastewater mixed with agricultural drainage. M. reisseri was selected based on the growth rate and cultivated in municipal wastewater (influent, secondary and tertiary effluents) which varied in nutrient concentration. M. reisseri showed an optimal specific growth rate (μ_opt) of 1.15, 1.04, and 1.01 1/day for the influent and the secondary and tertiary effluents, respectively. Secondary effluent supported the highest phosphorus removal (94%) and saturated fatty acid content (40%). The highest lipid content (40%), unsaturated fatty acid content, including monounsaturated and polyunsaturated fatty acids (66%), and nitrogen removal (80%) were observed for tertiary effluent. Fatty acids accumulating in the microalgal biomass (M. reisseri) were mainly composed of palmitic acid, oleic acid, linoleic acid, and a-linolenic acid. Cultivation of M. reisseri using municipal wastewater served a dual function of nutrient removal and biofuel feedstock generation.     

Simultaneous nutrient removal and lipid production from pretreated piggery wastewater by Chlorella vulgaris YSW-04

The feasibility of using a microalga Chlorella vulgaris YSW-04 was investigated for removal of nutrients from piggery wastewater effluent. The consequent lipid production by the microalga was also identified and quantitatively determined. The wastewater effluent was diluted to different concentrations ranging from 20 to 80 % of the original using either synthetic media or distilled water. The dilution effect on both lipid production and nutrient removal was evaluated, and growth rate of C. vulgaris was also monitored. Dilution of the wastewater effluent improved microalgal growth, lipid productivity, and nutrient removal. The growth rate of C. vulgaris was increased with decreased concentration of piggery wastewater in the culture media regardless of the diluent type. Lipid production was relatively higher when using synthetic media than using distilled water for dilution of wastewater. The composition of fatty acids accumulated in microalgal biomass was dependent upon both dilution ratio and diluent type. The microalga grown on a 20 % concentration of wastewater effluent diluted with distilled water was more promising for generating high-efficient biodiesel compared to the other culture conditions. The highest removal of inorganic nutrients was also achieved at the same dilution condition. Our results revealed the optimal pretreatment condition for the biodegradation of piggery wastewater with microalgae for subsequent production of high-efficient biodiesel.     

Suitability of Dicloster acuatus MH013435 and Kalenjinia gelatinosa MH012185 for growth in Garcinia cambogia washwater effluent with efficient nutrient removal

Abstract

The present study investigates the suitability of growing Dicloster acuatus MH013435 and Kalenjinia gelatinosa MH012185 in Garcinia cambogia washwater for biomass production and nutrient removal. In recent years, the wastewater effluent treatment with microalgae is gaining importance since it serves multiple purposes including CO₂ sequestration, production of biofuel feedstock and value-added biochemicals in addition to wastewater treatment. Contaminated washwater is generated at a significant quantity during extraction of hydroxycitric acid from G. cambogia and it represents a serious environmental concern when discharged without proper processing. However, this G. cambogia washwater can be used as a low-cost source of nutrients for microalgal growth. The two microalgal species D. acuatus MH013435 and K. gelatinosa MH012185 demonstrated robust growth in washwater and achieved maximum biomass concentration of 0.68 and 0.63?g/L dry weight. The nitrate was removed to an extent of 98.5 and 99.6% whereas chloride removal was 72 and 80.5%, sulfate concentration got reduced by 98 and 98.7% and phosphate got reduced to 95 and 92% for D. acuatus MH013435 and K. gelatinosa MH012185, respectively. The results indicate the suitability of D. acuatus MH013435 and K. gelatinosa MH012185 in treating G. cambogia washwater with efficient nutrient removal.     

微藻在废水处理和生物质回收再利用方面的研究进展

随着经济的发展和人口的增加,环境污染和水资源短缺已经成为不可避免的全球性问题。基于微藻的废水处理技术不仅可以净化废水、解决环境污染问题,还可以利用废水中的营养元素合成生物质,现如今这种技术已经受到越来越多的关注。为了进一步提高废水处理效果、降低废水处理成本,有必要了解微藻去除废水中营养物质和污染物的机理,开发下游低成本收获技术,提升微藻高价值副产物的生产。本文综述了微藻去除碳、氮、磷、重金属、抗生素和有机物的机理和影响因素,总结了微藻的不同收获方式和微藻生物质在各个领域的应用。最后,分析了不同微藻共培养体系和微藻固定化技术的优缺点,并展望了微藻废水处理技术未来的发展方向。     

Effect of intermittent CO2 enrichment during nutrient starvation on tertiary treatment of wastewater by alginate-immobilized Scenedesmus bicellularis

A method of accelerating the removal of ammonium and phosphate by the unicellular microalga Scenedesmus bicellularis is presented for municipal tertiary wastewater treatment using immobilized cells to obtain a high quality of effluents. Microalgal cells grown in defined medium were harvested by centrifugation and stored at 4°C in the dark for 8 months before immobilization. The concentrated cell suspension was then immobilized in alginate films supported on polypropylene screens. Immobilized cells were incubated in a water-saturated air stream enriched with CO₂ at 750, 1,000, or 1,500 ppm for 3 h periods followed by 2 h periods without enrichment. The quantitative effects of these three CO₂ enrichments on nutrient uptake from secondary municipal wastewater effluent were compared to a control laboratory air at 320 ppm under the same conditions of illumination, photoperiod, and humidity. The exposure cycle of 48-h nutrient deprivation in air with CO₂ enrichment followed by 2 h of nutrient uptake from wastewater was repeated three times with a residual NH₄---N content dropping to 0% after 105 min for the 1,500 ppm CO₂ treatment and to 34% of the initial level after 120 min for the control treatment. Complete PO₄---P removal required more than 2 h. The chlorophyll a contents obtained with 1,000 and 1,500 ppm CO₂ enrichments were comparable. This study establishes that intermittent CO₂ enrichment during nutrient deprivation of immobilized microalgal cells in a water-saturated air stream may accelerate tertiary wastewater treatment.     

Converting carbon dioxide to high value-added products: Microalgae-based green biomanufacturing

Today's world faces the dual pressure of carbon dioxide (CO₂) emission reduction and an energy crisis. Microalgae, which can use solar energy to convert CO₂ to organic matter, have emerged as a promising and renewable cell factory for producing nutrients, biofuels, and various high value-added compounds (HVACs). They possess numerous advantages, such as high photosynthetic efficiency, fast growth rate, and use of agro-industrial waste and nonagricultural land for cultivation. Microalgae can also effectively remove eutrophic elements (e.g., nitrogen and phosphorus) from wastewater and atmospheric pollutants (e.g., SO_x and NO_x) from flue gas, thus providing great environmental benefits. However, microalgae-based production often faces low productivity, limiting applicability in industrial settings. Genetic and metabolic modifications of certain microalgal strains have proven effective in improving productivity. Here, we review the latest developments regarding the microalgae-based production of platform compounds, biofuels, and other HVACs. Although still in the early exploration stage, the rapid development of gene editing tools, a deeper understanding of the metabolic pathways of microalgae and their regulatory mechanisms, and further optimization of cultivation procedures and photosynthetic efficiency can eventually enable the launch of microalgae-based biomanufacturing for green industrial production. Therefore, this technology is strategically important for solving the current energy crisis problems of excessive CO₂ emissions and environmental pollution. This review provides information about the advancement and development of microalgae-based production over the past two decades and discusses possible future directions in the field.     

Integration of microalgae cultivation with industrial waste remediation for biofuel and bioenergy production: opportunities and limitations

There is currently a renewed interest in developing microalgae as a source of renewable energy and fuel. Microalgae hold great potential as a source of biomass for the production of energy and fungible liquid transportation fuels. However, the technologies required for large-scale cultivation, processing, and conversion of microalgal biomass to energy products are underdeveloped. Microalgae offer several advantages over traditional 'first-generation' biofuels crops like corn: these include superior biomass productivity, the ability to grow on poor-quality land unsuitable for agriculture, and the potential for sustainable growth by extracting macro- and micronutrients from wastewater and industrial flue-stack emissions. Integrating microalgal cultivation with municipal wastewater treatment and industrial CO(2) emissions from coal-fired power plants is a potential strategy to produce large quantities of biomass, and represents an opportunity to develop, test, and optimize the necessary technologies to make microalgal biofuels more cost-effective and efficient. However, many constraints on the eventual deployment of this technology must be taken into consideration and mitigating strategies developed before large scale microalgal cultivation can become a reality. As a strategy for CO(2) biomitigation from industrial point source emitters, microalgal cultivation can be limited by the availability of land, light, and other nutrients like N and P. Effective removal of N and P from municipal wastewater is limited by the processing capacity of available microalgal cultivation systems. Strategies to mitigate against the constraints are discussed.     

Surface adsorption,intracellular accumulation and compartmentalization of Pb(II) in batch-operated lagoons with Salvinia minima as affected by environmental conditions,EDTA and nutrients

The effects of environmental factors and nutrients on the various possible removal mechanisms (surface adsorption, intracellular accumulation and precipitation to sediments) and partitioning of lead among various compartments (plant biomass, water column and sediments) in Salvinia minima batch-operated lagoons, were evaluated. Surface adsorption was found to be the predominant mechanism for Pb(II) removal under all environmental conditions tested in the absence of nutrients (an average of 54.3%) and in a nutrient medium (modified Hutner 1/10 medium) free of EDTA and phosphates (54.41%) at “high” initial Pb(II) concentrations (in the range of 10.3±0.13 to 15.2±0.05 mg/L). Under these conditions, the bioconcentration factors (BCFs) were 2,431±276 and 2,065±35, respectively. Lead removal was very rapid during the first 4 h and reached 70% in the absence of nutrients at the “medium” light intensity and temperature (LIT) tested, 88% in nutrient medium free of EDTA and supplemented with synthetic wastewater (at the “lowest” LIT tested), and 85% in medium free of EDTA and phosphates. It was concluded that the mechanisms of lead removal by S. minima, and the compartmentalization of this metal in the microcosm of batch-operated lagoons, are primarily a function of the presence of certain nutrients and chelants, with secondary dependence on environmental conditions. In addition, the results indicate that the percentage of lead removed is only a gross parameter and that the complementary use of BCF and compartmentalization analysis is required to gain a full insight into the metal removal process.

     

Performance of an ecological treatment system at three strengths of dairy wastewater loading

Ecological treatment systems, which rely on renewable resources, have successfully treated municipal and industrial effluents with reduced costs compared to conventional methods, but their capacity to treat dairy wastewater is unknown. In order for ecological treatment systems to be practical for agriculture they must be able to treat a significant portion of a dairy's daily wastewater production. In this study, the impact of three strengths of dairy wastewater on effluent water quality was assessed. Three ratios of wastewater and city water—(1) one part wastewater:three parts city water, (2) one part wastewater:one part city water, and (3) two parts wastewater:one part city water—were each pumped into an ecological treatment system. Influent and effluent water samples were analyzed for PO₄-P, TP, TN, NH₄-N, NO₃-N, total suspended solids (TSS), and carbonaceous biochemical oxygen demand (CBOD₅). Influent dairy wastewater volumetric loading rates were much greater than those of municipal wastewater. Regardless of influent wastewater strength, concentrations of all measured variables were significantly reduced between the influent and effluent of the ecological treatment system. At the lowest wastewater strength, PO₄-P was reduced 39%, TN 83%, and NH₄-N 89%, while at the highest wastewater strength, PO₄-P was reduced 41%, TN 79%, and NH₄-N 70%. Increased wastewater strength required greater aerobic treatment volume to reduce concentrations of NH₄-N and CBOD₅.     

Current research trends on microplastic pollution from wastewater systems: a critical review

Microplastics have been widely considered as contaminants for the environment and biota. Till now, most previous studies have focused on the identification and characterization of microplastics in freshwater, sea water, and the terrestrial environment. Although microplastics have been extensively detected in the wastewater, research in this area is still lacking and not thoroughly understood. To fill this knowledge gap, the current review article covers the analytical methods of microplastics originating from wastewater streams and describes their sources and occurrences in wastewater treatment plants (WWTPs). Studies indicated that microplastic pollution caused by domestic washing of synthetic fibers could be detected in the effluent; however, most microplastics from personal care and cosmetic products (PCCPs) can be efficiently removed during wastewater treatment. Moreover, various techniques for sampling and analyzing microplastics from wastewater systems are reviewed; while, the implementation of standardized protocols for microplastics is required. Finally, the fate of microplastics during wastewater treatments and the environmental contamination of effluent to environment are presented. Previous studies reported that the advanced wastewater treatment (e.g., membrane bioreactor) is needed for improving the removal efficiency of small-sized microplastics (<?100 µm). Although the role of microplastics as transport vectors for persistent organic pollutants (POPs) is still under debate, they have demonstrated abilities to absorb harmful agents like pharmaceuticals.

     

The potential of sustainable algal biofuel production using wastewater resources

The potential of microalgae as a source of renewable energy has received considerable interest, but if microalgal biofuel production is to be economically viable and sustainable, further optimization of mass culture conditions are needed. Wastewaters derived from municipal, agricultural and industrial activities potentially provide cost-effective and sustainable means of algal growth for biofuels. In addition, there is also potential for combining wastewater treatment by algae, such as nutrient removal, with biofuel production. Here we will review the current research on this topic and discuss the potential benefits and limitations of using wastewaters as resources for cost-effective microalgal biofuel production.     

Effect of nitrogen and phosphorus concentration on their removal kinetic in treated urban wastewater by Chlorella vulgaris

This study evaluates the feasibility of removing nutrients by the microalgae Chlorella vulgaris, using urban wastewater as culture medium, namely the effluent subjected to secondary biological treatment in a wastewater treatment plant (WWTP). For this, laboratory experiments were performed in batch cultures to study the effect of initial nitrogen and phosphorus concentrations on growth and reduction of nutrient performance of C. vulgaris. The microalga was cultivated in enriched wastewater containing different phosphorus (1.3-143.5 mg x L(-1) P.PO4(3-)), ammonium (5.8-226.8 mg x L(-1) N-NH4+) and nitrate (1.5-198.3 mg x L(-1) N-NO3-) concentrations. The nutrient removal and growth kinetics have been studied: maximum productivity of 0.95 g SS x L(-1) x day(-1), minimum yield factor for cells on substrate (Y) of 11.51 g cells x g nitrogen(-1) and 0.04 g cells x g phosphorus(-1) were observed. The results suggested that C. vulgaris has a high potential to reduce nutrients in secondary WWTP effluents.     

Phosphorus removal coupled to bioenergy production by three cyanobacterial isolates in a biofilm dynamic growth system

In the present study a closed incubator, designed for biofilm growth on artificial substrata, was used to grow three isolates of biofilm-forming heterocytous cyanobacteria using an artificial wastewater secondary effluent as the culture medium. We evaluated biofilm efficiency in removing phosphorus, by simulating biofilm-based tertiary wastewater treatment and coupled this process with biodiesel production from the developed biomass. The three strains were able to grow in the synthetic medium and remove phosphorus in percentages, between 6 and 43%, which varied between strains and also among each strain according to the biofilm growth phase. Calothrix sp. biofilm turned out to be a good candidate for tertiary treatment, showing phosphorus reducing capacity (during the exponential biofilm growth) at the regulatory level for the treated effluent water being discharged into natural water systems.

Besides phosphorus removal, the three cyanobacterial biofilms produced high quality lipids, whose profile showed promising chemical stability and combustion behavior. Further integration of the proposed processes could include the integration of oil extracted from these cyanobacterial biofilms with microalgal oil known for high monounsaturated fatty acids content, in order to enhance biodiesel cold flow characteristics.     

A comparative study of four systems for tertiary wastewater treatment by Scenedesmus bicellularis: New technology for immobilization

Immobilization appears to be one of the best techniques to separate physically micro-algal cells from their culture medium for the purpose of algal tertiary wastewater treatment. High operation costs and other drawbacks of large-scale physico-chemical methods of harvest led to a comparative study of biotreatment systems. Before treatment began, Scenedesmus bicellularis cells were conditioned (starved) under four different sets of conditions: 1) non-immobilized cells with air bubbling (NCA); 2) cells immobilized in alginate beads (CBW) and 3) cells immobilized on alginate screens (CSW), all conditioned in synthetic culture medium depleted in N and P; 4) cells immobilized on alginate screens but conditioned in air at 100% relative humidity (CSA). Starvation was started under a light:dark photoperiod of 16:8 h. Starved cells were then used to treat wastewater for a 2-h period. The performance of each system was evaluated by determination of residual NH₄-N and phosphate ions and by growth (dry weight, total chlorophyll, cell count, protein content). We then tested the capacity of microalgae immobilized on screens to eliminate N and P from a secondary municipal wastewater effluent and examined the influence of temperature and starvation. The quality of treated effluents was improved considerably with the system using CSA or CSW model. For CSA model, the protein content was 22.4 pg cell^-1 compared to 12.9, 9.5, 9.1 pg cell^-1 for NCA, CBW and CSW models, respectively. The CBW and CSW models were efficient for chlorophyll synthesis. The residual ammonium content in natural wastewater after 2 h of treatment with CSA model was 39% at 6±2 °C and reached 100% removal at 18±2 °C. With the first 2 h, the removal of orthophosphate was inferior (53%) at 6±2 °C, but 88 to 100% at 18±2 °C depending on starvation times. Long starvation times (72 or 96 h) caused damage to cells and uptake of nutrients was lower than with 54 h starvation. This work demonstrates that by using immobilization on screens, removal of nutrients from wastewater was higher than with conventional biological tertiary wastewater treatments (free cells or bead-shaped alginate particles).     

The effect of managing nutrients in the performance of anaerobic digesters of municipal wastewater treatment plants

Is it possible to create conditions in the anaerobic digesters to control nutrients without changing the performance of a reactor? This study investigates an answer for this question. To this purpose, anaerobic reactors are operated at high concentrations of Mg²⁺ ion to harvest the nutrient ions (NH₄ ⁺ and PO₄ ^3?) in the form of struvite, that is, magnesium ammonium phosphate. The effects of this modification on the anaerobic digestion of sewage sludge were investigated in terms of chemical oxygen demand (COD) removal and cumulative CH₄ production as well as the changes in the biological diversity. The results showed that approximately 50 % of the nutrients (NH₄ ⁺ and PO₄ ^3?) were removed regardless of the method adopted for the addition of Mg²⁺ ion, slug or daily dosing. The numbers of Methanosaeta and Methanosarcina in the samples withdrawn prior to and after the addition of Mg²⁺ did not show significant difference according to the results obtained from qPCR analyses. The research results showed that the addition of Mg²⁺ into the anaerobic digesters in municipal wastewater treatment facilities may help to remove the nutrients from the effluent while recovering in their solid forms.     

Modelling pollutant removal in a pilot-scale two-stage subsurface flow constructed wetlands

The hydraulic behaviour and effluent pollutant concentrations in a pilot-scale two-stage subsurface flow constructed wetland for treatment of municipal wastewater have been simulated. The experimental pilot plant is located in San Michele di Ganzaria (Eastern Sicily) and consists of four lines of two-stage subsurface flow constructed wetlands for secondary or tertiary treatment of municipal wastewater. The first stage, for each line, consists of a horizontal flow bed, while in the second stage a vertical flow bed operates for two lines and a horizontal flow bed for the other two. Phragmites sp. was used as vegetation in two lines while the other two lines are without plants. The HYDRUS-2D software was applied to describe flow and single-solute transport, while the multi-component reactive transport module CW2D was used to model the transformation and elimination processes of organic matter, nitrogen and phosphorus. Tracer studies and chemical wastewater analyses were carried out to calibrate and validate the transport model. In general, the simulation results obtained show a good match with the measured data for water flow, tracer experiments and pollutant removal processes.     

The Integrated System of Phytodepuration of Sile River Natural Park

The water conservation topic is likely to become increasingly important and alternative water resources employment should be considered as one possible response to the challenges of fresh water demand and environmental protection; among alternative water sources, municipal wastewaters represent one of the most profitable source but in order to reuse them they need adequate and advanced depuration techniques, such as the use of Integrated System of Phytodepuration (ISP). Across a 3-year sampling period, the performances of an ISP within the Natural Park of the Sile River in the Northern Italy were evaluated, analyzing raw wastewater and final effluent characteristics according to the recommendations of European and Italian legislation. The investigated ISP represents one of the first attempts designed in Italy to improve the efficiency of an existing wastewater treatment plant, able to serve 8000 equivalent inhabitants. The results obtained during the 3 years of analysis show that the designed ISP is characterized by a general efficiency value higher than 87% for TSS removal, 79% for TN, 91% for BOD₅ and 86% for COD; moreover the ISP final effluent is characterized by a quality not only suited for release into surface waters but also for irrigation.     

Moving forward in the use of aerobic granular sludge for municipal wastewater treatment: an overview

Activated sludge is one of the most widely implemented technologies for municipal wastewater treatment. Yet, more restrictive environmental standards demand for more efficient technologies. Aerobic granular sludge (AGS) is a promising alternative in this context since this technology has shown potential for simultaneous organic matter and nutrient removal using smaller bioreactors and consuming less energy. However, despite such engaging claims, only ca. 40 full-scale AGS systems have been installed worldwide after 30 years of development. This reduced implementation suggests the existence of significant bottlenecks for this technology, which currently only have partially been overcome. This overview aims to analyze the recent progress in R&D concerning aerobic sludge granulation for municipal wastewater treatment via the analysis of research articles and invention patents as well as to elucidate exiting technological gaps and development opportunities. Culturing methods aiming at fast granulation, long-term stability and excellent process performance are of utmost interest for promoting massive implementation of full-scale AGS systems. Moreover, the recovery of biomaterials from waste sludge could contribute to the implementation of the biorefinery paradigm in wastewater treatment plants.

     

Simultaneous removal of inorganic nutrients and organic carbon by symbiotic co-culture of <Emphasis Type="Italic">Chlorella vulgaris</Emphasis> and <Emphasis Type="Italic">Pseudomonas putida</Emphasis>

The co-culture system of photosynthetic microalgae Chlorella vulgaris and aerobic heterotrophic bacteria Pseudomonas putida was investigated as a possible combination of symbiotic mixed culture for the simultaneous removal of nutrients (ammonium and phosphate) and organic contaminants. Using synthetic municipal wastewater, the co-culture system exhibited symbiotic enhancement in the removal of nutrients and organic carbon compared to each of axenic cultures. The co-culture system performed successfully in removing both of ammonium and chemical oxygen demand (COD), showing around 80% removal for 4 days. Strategies of nitrogen and phosphorous starvation in C. vulgaris for two days prior to main treatment did not increase the performance of nutrients removal, indicating that the nutrient starvation as a pretreatment is unnecessary. Without alkalinity (as bicarbonate), nutrients and COD were not removed significantly, implying that the existence of alkalinity is essential for symbiotic treatment of both nutrients and organics. Results demonstrated that coculture system composed of C. vulgaris and P. putida can be a potential candidate of mixed culture system for the simultaneous removal of nutrients and organic carbon in wastewater treatment using a single reactor.     

Nutrient removal and biomass production in land treatment systems receiving domestic effluent

Land application of pre-treated wastewater is increasingly practiced to achieve both treatment and beneficial reuse of applied effluent. Vegetation is an important component of these systems, affecting hydraulic loading and nutrient uptake and hence treatment efficiency. This work investigated the effect of plant species (Acacia cyanophylla, Eucalyptus camaldulensis, Populus nigra and Arundo donax), on water requirements, nutrient removal, water use efficiency (WUE) and biomass production in land treatment systems (LTS) in which pre-treated wastewater was applied at rates to meet crop evapotranspiration. Vegetation had a strong effect on all the parameters monitored during this trial. A. cyanophylla produced the greatest amount of biomass and showed the highest irrigation requirements and WUE, followed by E. camaldulensis, A. donax and P. nigra. In addition, A. cyanophylla and A. donax achieved a higher leaf-N content compared to other species. As a result of the differences in tissue nutrient content and biomass, A. cyanophylla accumulated 23, 20, and 70% more N in hypergeous biomass than E. camaldulensis, A. donax and P. nigra, respectively. A. cyanophylla and E. camaldulensis accumulated 57 and 53% respectively more P than did P. nigra and A. donax. Therefore substantial improvement of the performance of the LTS in terms of nutrient removal can be achieved through the selection of appropriate plant species. Despite the enhanced growth rates observed in the study nutrient recovery by vegetation did not exceed 31 and 35% of the applied N and P, respectively. The relatively low percentages of removal are attributed to increased concentration of nutrients in effluent and the high ET rates prevailing in the study area. These findings suggest that additional practices are required to mitigate environmental impacts arising from excessive nutrient loads when effluent is applied at rates to meet crop water requirements.