Investigation of a Case of Failure of a Reverse Osmosis Project

By

M. Gamal Khedr

Professor Emeritus, National Research Centre, Cairo

mgakhedr@yahoo.fr

 

Abstract

A reverse osmosis (RO) project of 22,000 m3/d was installed for desalination of a groundwater contaminated by the radioactive isotopes radium (Ra 226, 228) to produce drinking water in Qaseem region, Saudi Arabia. The project specified the blending of RO permeate of 20,000 m3/d with 2,000 m3/d of conditioned raw water. Since the radioactive contamination is rather high (195.1 ± 20.2 pCi/L) and it would further increase with time upon withdrawal of groundwater and since, on the other hand, the RO rejection would decrease with time during the membrane lifetime, the project included, as a pretreatment process prior to RO, the partial removal of Ra by adsorption on the active hydrous manganese oxide (HMO) surface and filtration.

 However, upon initial start up the plant failed to realize the tender requirements of both lowering of radioactivity up to the maximum contaminant level (MCL) in drinking water   of ≤ 5 pCi/L as per the norms of the US Environmental Protection Agency, as well as of rejection of salinity by RO to ≤ 500 mg/L according to the norms of the World Health Organization (WHO).

Our consultancy investigation revealed erroneous application of the technology of Ra removal by adsorption on HMO. This lowered Ra rejection, by this process, to only 57.4% while the project book specified a value of 90%. On the other hand, salt rejection by RO was only 88% instead of 96% as per the project description. These deffects resulted in a product water salinity and Ra contamination out of the requested values upon blending the RO permeate with the conditioned raw water.  The consultancy investigation rectified the process of preparation of the active HMO layer which realized the expected Ra adsorption and removal by subsequent filtration. RO performance was also improved by modification of the system design of the RO skids and adopting a developed array according to a standard software projection to obtain salt rejection of 96%. Pilot testing confirmed the satisfaction of the tender conditions.

 

Keywords: Groundwater Treatment, Radium contamination, Hydrous Manganese Oxide, Preparation of HMO, Adsorption on HMO.

                                                    Introduction

Selective membrane processes as RO and nanofiltration (NF) are currently commonly approved as the most efficient and cost effective methods for desalination of brackish groundwater or industrial waste waters of medium salinity (1). On the other hand, RO process was reported to reject efficiently the divalent radioactive ionic species as radium (Ra2+ 226,228) and heavy metal cations (2, 3).

The present project faces the double challenge of a rather high radioactive contamination by Ra 226,228 of 195.1 ± 20.2 pico Curie/L (pCi/L), parallel to too high salinity than suitable for drinking water of 3538.4 mg/L. In view of the similar properties of Ra and Ca, both being alkaline earth metals, consumption of Ra contaminated drinking water would lead to bone deformation, bone cancer and other dangerous attacks then eventual death (4). The present project selected, therefore, the application of RO preceded by a specific pretreatment for partial rejection of radioactivity.

Aim and scope of the project:

  1. Production of drinking water of total dissolved solids (TDS) ≤ 500mg/L according to the norms of the World Health Organization (WHO) (5).
  1. Lowering of the Ra radioactivity to ≤ 5pCi/L according to the norms of the US-Environmental Protection Agency (EPA) (6).
  1. In view of the rather high detected radioactive contamination and the risk of its possible increase with time upon withdrawal of groundwater, and as precaution against the possible decrease of RO rejection during the membrane lifetime, the project book specified partial rejection of Ra by adsorption on the activated MnO2 surface and filtration prior to RO as a double safety barrier against the contamination of drinking water. Hydrous manganese oxide (HMO) on the surface of activated MnO2 is known to adsorb efficiently Ra and other divalent or polyvalent metal cations, under critical specific conditions, and is the basis of a standard method for Ra removal (7).

However, upon the start up of the project neither the specified salt rejection nor the Ra rejection were realized. The present consultancy investigation aims to discover the sources of this failure and propose the adequate rectification for the realization of the project specified performance.

                                               Discussion

Project Facts:

Plant Capacity:

Final product rate of 22,000 m3/d drinking water of which 20,000 m3/d are to be produced by RO then blended with 2,000 m3/d conditioned raw water.

Raw Water Quality:

Groundwater analysis is given by Table 1.

The radioactive contamination is due to combined Ra 226,228 of 195.1 ± 20.2 pCi/L.

Tender Described RO System and Performance:

RO membranes of the modern technology of thin film composite, spiral wound membrane elements of the polyamide chemistry (TFC, SW, PA).

Salt rejection of RO units not ˂ 96%

Percent recovery of 85%

Permeate Flux not >20 lmh

 Investigation of the Failure of the Partial Removal of Ra:

 The project specified the pretreatment of the groundwater for Ra removal by adsorption on freshly prepared surface active layer of HMO then filtration. However, the resultant removal of Ra by this step was quite lower than expected and led to unacceptable contamination of the final drinking water. If we consider the main lines of the procedure of removal of Ra by adsorption on HMO layer:

  1. MnO2 is prepared according to the reaction:

  1. The treatment should start by oxidation of the dissolved iron and manganese from the raw water and filtration for removal of the corresponding oxides to prevent the competitive adsorption of these cations on the HMO layer which lowers Ra adsorption (8).
  1. Upon oxidation of iron and manganese in the aeration towers a part of Ra is, in fact, removed by adsorption on their deposited oxides (9).
  1. The reagents MnSO4 and KMnO4 should separately be dissolved in agitated tanks then mixed in a third tank with continuous agitation.
  1. Since deposition of MnO2 is accompanied by production of H2SO4, an alkali as caustic soda should be dosed to neutralize the pH to the range 8-8.5.
  1. At this pH, a contact time of 10 to 12hrs with agitation is required for deposition of MnO2 and for its surface hydrolysis.
  1. Then the resulting suspension is dosed to the feed stream. Usually a dose of 4 mg/L is used.

 

Analysis of the failure of Ra removal by HMO:

Inspection of the system of preparation of HMO revealed the following defects:

  1. The above described standard procedure for preparation of the HMO was not respected. The dissolved reagents KMnO4 and MnSO4 instead of being mixed together and left over the mentioned contact time for deposition of MnO2 and surface hydrolysis prior to dosing to the raw water stream, they were directly independently added to the feed water stream.
  1. These reagents were, therefore, much diluted before reaction without the correct contact time at the suitable pH for the adequate formation of the active layer.
  1. The added concentrations of these reagents were found to be lower than standard as per equation [1]. According to the operation team this was to prevent the repeated tripping of the plant.

In fact, plant tripping was due to the oxidizing alert caused by the unreacted KMnO4 which raised the oxidation-reduction potential (ORP) in the feed stream above the limit fixed for protection of the PA membranes. The operation decreased the dosing of the oxidizing KMnO4 to avoid the repeated tripping.

 

These problems explain the incomplete reaction of formation of the HMO layer and consequently the observed failure of Ra removal.

While the process flow diagram of the project tender specified 90% Ra rejection, the feed water radioactivity of 195.1 ± 20.2 pCi/L was lowered to 83.3 ± 8.9 pCi/L i.e. by only 57.4%.

Investigation of the failure of RO units to realize the tender requirements of salt and radioisotope rejection.

 

The project four (4) RO skids of 5,000 m3/d each were based on 8 inch, TFC/SW/PA membranes according to an RO system design that enabled rejection of TDS to 401.5 mg/L i.e. by only 88,27 % and Ra rejection to 5.4 ± 1.3 pCi/L  while the project specified RO salt rejection of ≥ 96%.

The consultant investigation had to upgrade Ra rejection from the feed water by correct preparation of the active adsorbing HMO layer in order to enable safe values of radioactivity upon blending with the RO permeate.

When the preparation of the active layer was rectified according to the described standard procedure, a 92.6 % rejection of Ra was realized to give a residual activity in the RO feed of only 14,48 pCi/L.

As for RO Rejection, on the other hand, we have developed, based on RO system design projections using a standard software, a different skid design Fig 1 which resulted in permeate TDS of 124.06 i.e. at 96.5 % salt rejection and practically complete Ra the rectified rejection (for blending calculations we have selected the highest reading), Fig 2.

Pilot testing of Ra removal by adsorption on the rectified HMO layer, and of salt rejection by the modified RO unit design confirmed the realization of the project tender specifications upon blending as follows:

Conclusions

 

  1. A consultancy investigation was conducted for a case of failure of an RO plant which treats groundwater contaminated by the radioactive isotopes Ra 226,228 at an activity quite higher than the permissible MCL in drinking water. The primary inspection revealed that both TDS and radioactivity of the final product water unacceptably exceed the project specifications.
  1. While selective membrane techniques as RO or NF show efficient rejection of divalent and polyvalent ionic species as radioisotopes and heavy metal cations, in case of high concentrations of such dangerous contaminants RO can be successfully combined with other decontamination processes as a double safety barrier to realize drinking water norms.
  1. The pretreatment adsorption of Ra on the adequately prepared active HMO and filtration could realize > 90 % removal of Ra from raw water. Preparation includes mixing of the reagents KMnO4 and MnSO4 with agitation and pH adjustment to the range (8-8.5) over a contact time of 10 hours then dosing to the main stream.
  1. The Ra bind to HMO in the back wash stream of the media filters together with the RO reject stream go to the evaporation ponds where the radioactivity is diluted in the solid state by the rejected salts and goes to the adequate disposal.

References

  • Gamal Khedr, “Optimization of reverse osmosis process efficiency and Environmental safety through reject processing”, Euromembrane International Conference, Hambourg (2004) 600.
  • Gamal Khedr, “Nanofiltration and low energy reverse osmosis for rejection of radioactive isotopes and heavy metal cations from drinking water sources”, Desalination and Water Treatment, 2 (2009) 342 – 350.
  • J. Sorg et al, “Removal of Ra 226 from Sarasota County, Flo., drinking water by reverse osmosis”, J. Am. Water Works Assoc., 72 (4) (1980) 230.
  • M. Finkelsteing and N. Kreiger, “Radium in drinking water and risk of bone cancer in Ontario youths: a second study and combined analysis”, Occup. Environ. Med., 53 (5) (1996) 305 – 11.
  • Drinking-Water Quality Guidelines-WHO, https://www.who.int>guidelines 4th edition (2017).
  • The Radionuclides Final Rule, Environmental Protection Agency, https://www.epa.gov.>dwreginfo>radio December 2000.
  • A. Clifford, “Radon, Radium and Uranium in Drinking Water”, C.R. Cothern & A.P. Rebers, Eds., Lewis Publishers, (1991) 234
  • L. Brink et al, “Radium removal efficiencies in Water Treatment Processes”, J. Am. Water Works Assoc., 70 (1) (1978) 31.
  • Clifford et al, “Evaluating various adsorbents and membranes for removing radium from groundwater”, J. Am Water Works Assoc., (1988) 94 – 104.

 الإسموزية والضغط الإسموزي  (محاولة تفسير وفهم لظاهرة فيزيائية )

عند الوصول إلي حالة الإتزان ، فإن جانب المحلول الأعلي في التركيز يحصل علي ارتفاع أكبر في منسوب المحلول من جانب المحلول الأقل في التركيز رغم تساوي ارتفاع المحلولين علي جانبي الغشاء في البداية.إن الإسموزية هي عملية طبيعية تلقائية ديناميكية وفيها تنتقل جزيئات الماء أو المذيب خلال غشاء شبه منفذ من الوسط الأقل إلي الوسط الأعلي في تركيز المواد الذائبة لتعادل تركيز المحلولين علي جانبي الغشاء . وهي عملية مهمة وتلعب دورا متكاملا في كثير من الأنظمة البيولوجية ، والأنسجة الحية من أجل الحفاظ على بيئة داخلية مثالية ، وكذلك إنتقال المياه إلي النبات من التربة عبر أغشية الجذر وهو مهم لعملية البناء الضوئي وغيره من العمليات الحيوية الهامة.

هذا الفرق في الإرتفاع الناتج بين المحلولين علي جانبي الغشاء شبه المنفذ عند الإتزان هو ما يسمي بالضغط الإسموزي ، وبمعني أخر هو الضغط الخارجي المطلوب لمنع تدفق المياه بين المحلولين المختلفين في التركيزعبر الغشاء شبة المنفذ.

والسؤال الواضح الذي يطرح نفسه هو كيف يمكن أن يتحرك المذيب عبر غشاء شبه منفذ من منطقة ذات ضغط تناضحي أقل إلى ضغط تناضحي أعلى متغلبا على الضغط الهيدروستاتيكي.

على سبيل المثال ، في الشكل الموضح أدناه ، يتم فصل ذراعي أنبوب عن طريق غشاء شبه نفاذ حيث يحتوي الذراع الايمن على محلول ذو تركيز منخفض من المذاب مقارنة بتركيز محلول ذراع الجانب الأيسر.

في البداية ، يكون المستوى هو نفسه في ذراعي الأنبوب . عندما يبدأ التناضح ، فإنه يدفع المذيب من ذراعه اليمني إلى ذراعه اليسرى عبر الغشاء شبه النفاذ ، حتى يساوي الضغط التناضحي في الذراع الأيمن مع الضغط الهيدروستاتيكي الذي يمارسه الذراع الأيسر.

وعلى الرغم من أن التناضح هو ظاهرة مألوفة ، وله أهمية محورية في النظم الطبيعية ، إلا أنه نادراً ما يفسر كيف يمكن أن يعمل على المستوى الجزيئي. وفي حين أن هناك العديد من النظريات والتفسيرات التي حاولت تفسير الخاصية الاسموزية ، فإن كل منها استطاع تفسير جزء من ظواهر تلك العملية وأخفق في تفسير جزء أخر ولا يوجد حتي الأن رؤية شاملة لماذا تحدث الخاصية الأسموزية ؟ وفي هذا المقال سيتم استعراض ومناقشة ثلاثة من أهم تلك التفسيرات: 1- الإنتشار بسبب التدرج المفترض لتركيز المياه ، 2- جزيئات المياه المقيدة ، 3- عدد الجسيمات أو ضغط البخار لفانت هوف .

الإنتشار بسبب التدرج المفترض لتركيز المياه

هو تفسير بسيط جدا للتناضح ، حيث أن جزيئات المياه تنتقل وتنتشر عبر التدرج في تركيزها من المحلول النقي حيث يكون الماء هو أكثر تركيزاً من المحاليل التي تحتوي علي مواد مذابة لأن المذاب يجب أن يأخذ بعض المساحة في المحلول فتخفيف الماء عن طريق المذاب يؤدي إلى انخفاض تركيز الماء ، وبالتالي يحدث انتشار للماء على طول التدرج في تركيزها من المرتفع إلى المنخفض .

يبدو تفسير جيد ، أليس كذلك ؟ لكن هناك خطأ ما.

فمثلا لو افترضنا أن المحلولين علي جانبي الغشاء أحدهما من الماء النقي والأخر محلول من السكر ، فلماذا لا تتدفق المياه بشكل عشوائي عبر مسام الغشاء في أي من الاتجاهين ، مما يؤدي إلى عدم وجود ضغط تناضحي؟

نعم ، تركيز الماء أقل على الجانب الذي به محلول السكر ، ولكن ماذا يعني ذلك لأي مسام فردية؟ إذا لم يكن جزيء السكر بالقرب من المسام ، فإن تركيز الماء هو نفسه على جانبي المسام ، وإذا كان جزيء السكر قريبًا بما يكفي من المسام ليحجب جزءاً منها ويمنع جزيئات الماء من الهروب ، فإنه يجب أن يمنع تدفق المياه في كلا الاتجاهين ، دون تمييز ، فقد تضرب جزيئات الماء التي تحاول اختراق المسام من الجانب الأخر جزيء السكر ، لكن جزيئات الماء في نفس الجانب تدفع أيضًا جزيء السكر.

أيضاً إذا كان هذا صحيحًا ، فيجب أن يكون تركيز الماء قادراً على التنبؤ باتجاه التناضح عند استخدام المذيبات المختلفة. على سبيل المثال ، يحتوي محلول السكروز(0.2 مولالي أي على 0.2 مول من السكر في كيلوغرام من الماء) على تركيز مائي يبلغ 937 جم / لتر ، بينما يحتوي محلول كلوريد الصوديوم (0.2 مولالي) على تركيز ماء أعلى بكثير – 989 جم / لتر.  ولذلك يجب أن يحصل محلول السكروز على الماء من محلول كلوريد الصوديوم إذا تم فصل الاثنين عن طريق غشاء نصف نافذ .

لكن الحقيقة هي أن المحلول الملولاي للسكروز مع إحتواءه علي تركيز ماء أقل من نفس تركيز المحلول المولالي لكلوريد الصوديوم يزيح المزيد من الماء في إتجاه محلول كلوريد الصوديوم لذلك ، لا يبدو أن تدرج تركيز الماء مهم.
لذلك هذا التفسير لا يمكن الدفاع عنه.

تفسير المياه المقيدة

هذا يعني أن أي مادة قابلة للذوبان في الماء (مثل السكروز أو كلوريد الصوديوم) سوف يرتبط بجزيئات المياه ويكون الجزئات أو الأيونات المماه ويمنع جزيئات الماء تلك من التحرك بحرية . لذلك ، فإن جانب غشاء شبه المنفذ والذي به بماء نقي له تركيز ماء “حر” أعلى من الجانب الذي به جزيئات ذائبة .

أو بطريقة أخري فإن المياه تنتقل من المحاليل المائية ذات الجهد الكيميائي الأكبر إلي المحاليل ذلت الجهد الكيميائي الأقل عند فصلهم بغشاء شبه منفذ ،حيث أن المياه النقية ذات جهد كيميائي أكبر.

ولكن إذا تحرك الماء في اتجاه من الجهد الكيميائي الأعلي إلى الجهد الكيمائي الأقل على أساس تركيز المذاب فقط ، فمثلا  إذا  كنت تفصل محلولين من ملح كلوريد الصوديوم والسكروز لهما نفس التركيز المولي بغشاء شبه منفذ ، يجب أن ينتقل الماء من جانب كلوريد الصوديوم (المول 58.5 جرام) حيث يوجد المزيد من الماء لجانب السكروز(المول 342 جرام) ، حيث يوجد ماء أقل. لكن هذا لا يحدث ، حيث يمتص محلول كلوريد الصوديوم الماء أكثر .

والواقع الفعلي هو أن التناضح لا يعتمد على درجة إماهة ولا حجم جزيئات المذاب بقدر ما يعتمد علي عدد جزيئات المذاب .

عدد الجسيمات أو قانون فانت هوف .

يستند هذا التفسير على قانون فانت هوف. وفقا لهذا القانون لمحلول مخفف في درجة حرارة ثابتة ، فإن الجهد التناضحي يتناسب مع تركيز وعدد جزيئات المذاب. وليس حجم أو طبيعة الجزيئات المذابة فقط . لذلك ، على سبيل المثال ، سيكون لأيون الصوديوم الصغير نفس التأثير التناضحي مثل جزيء السكروز الكبير ، وكلاهما سيعادل جزيء نشا كبير جدا. هذا يعني أيضا أن المواد المؤينة مثل كلوريد الصوديوم يجب أن يكون لها تأثير تناضحي أكبر من المواد غير المؤينة مثل السكروز لأنه عندما تتأين ، فإنها تولد المزيد من الجسيمات . قانون فانت هوف يفسر إلى حد ما التناضح بشكل أفضل من التفسرين السابقين ، ولكن الحقيقة هي أن قانون فانت هوف هو علاقة تجريبية ، وليس وصفاً مادياً لماذا يحدث التناضح.

π = iMRT

حيث ، π هو الضغط التناضحي للمحاليل

i هو معامل فانت هوف.

M = التركيز المولي mol / L.

R = الثابت العام للغازات  L · atm / mol    0.08206= K

T = درجة الحرارة المطلقة في K= 273 + °C))

ولذلك فإن الضغط التناضحي يعتمد على 1- التركيز المولي للمادة المذابة 2- عامل فانت هوف ، وهو يعبر عن مدى ترابط أو تفكك المواد الذائبة في المحلول أو عدد الجسيمات التي يتفكك فيها المذاب في الماء ، فمثلا السكروز = 1 وكلوريد الصوديوم = 2 .

مثال : ما هو الضغط التناضحي لمحلول 1.00 مولار من السكروز عند 25 درجة مئوية؟

عندما ندخل في المعادلة ، لدينا:

(π = i (1.00 mol / L) (0.08206 L atm / mol K) (298 K)

ومع ذلك ، هناك  مجهولين: π وهو ما نريد ، و i معامل فانت هوف

وهو ثابت تجريبي بدون وحدة يرتبط بدرجة تفكك المذاب ، وهذا يعني أنه مجرد رقم مثل 1 أو 2 يجب أن نحدده بالتجربة . ويمكنك التنبؤ بما يمكن أن تكون عليه القيمة النظرية ، ولكن القيمة الحقيقية لا توجد إلا في التجربة ، وهو يمثل “درجة التفكك”. وبذلك يكون معامل فانت هوف للسكروز هو 1 ، لأن السكروز لا يتأين في المحلول وتبقى جزيئات كاملة. وبالتالي فإن الجواب هو (24.4atm).

مثال أخر : ما هو الضغط التناضحي (عند 25 درجة مئوية) لمياه البحرتحتوي على حوالي 35.0 جرام من كلوريد الصوديوم لكل لتر. (تحتوي مياه البحر على أملاح أخرى ، ولكننا سنقوم بتجاهلها.)

نحول جرام إلى مول:

35.0 جم / لتر ÷ 58.443 جم/ مول = 0.599 مول / لتر

الآن ، قم بتوصيل المعادلة:

π = (i) (0.599 مول / لتر) (0.08206 لتر atm / mol K) (298 K)

ولكن ما  قيمة معامل فانت هوف لمحلول كلوريد الصوديوم؟

عندما يتأين كلوريد الصوديوم في المحلول ، فإنه ينتج أيونات الصوديوم والكلوريد. حيث ينتج مول واحد من كلوريد الصوديوم مول من كل نوع من الأيونات. لذا فإن عامل فانت هوف نظريا يساوي 2 ، ومع ذلك ، سنستخدم 1.8

لذا ، فان الحل:

π = (1.8) (0.599 mol/L) (0.08206 L atm / mol K) (298 K)

π = 26.4 atm

لماذا استخدم 1.8 لمعامل فانت هوف بدلاً من 2 ؟

هذا له علاقة بمفهوم يسمى الإقتران الأيوني . حيث أن عدد معين من أيونات الصوديوم وأيونات الكلوريد يجتمعان عشوائياً معًا ويكوّنا أيون كلوريد الصوديوم المزدوج . هذا يقلل من العدد الإجمالي للجسيمات في المحلول ، وبذلك يقلل معامل فانت هوف.

بقلم م/ يحيي علي

 

المصادر :

Water Desalination

Introduction

Only 1 percent of the earth’s water is liquid freshwater; 97 percent of available water resources are contaminated by salt. This makes desalination an essential component of efforts to address water shortages, especially in densely populated coastal regions. Egypt faces nowadays severe challenges to our ability to meet our future water needs, So we as a nation will need to make additional water resources available to all segments of our nation’s and provide additional water resources at a cost and in a manner that supports urban, rural and agricultural prosperity and environmental protection; Meeting these challenges may lead us to use saline water for a greater national focus on water conservation.

Desalination Definition

Desalination is a process that removes salts and other dissolved solids from brackish Water or seawater.

Brackish water and seawater

Brackish water is saltier than fresh water, but not as salty as seawater. Brackish water usually has a salt concentration between 5 and 20 parts per thousand (ppt) and seawater generally has a concentration of salt greater than 20 ppt. Brackish waters may also be found in aquifers.

Water type and Total Dissolved Solid:

TDS(mg/l) Water type
0-1000 Sweet waters
1000-5000 Brackish waters
5000-10000 Moderately saline waters
10000-30000 Severely saline waters
More than 30000 Seawater

Significance of Desalination

  • Desalination technologies will contribute significantly to ensuring a safe, sustainable, Affordable, and adequate water supply for Egypt.
  • Provide safe water: A safe water supply is one that meets all drinking water standards, meets all standards for use by agricultural and industrial interests, and that strives to move toward greater water security during drought, natural disasters, transport.
  • Ensure the sustainability of the nation’s water supply: A sustainable water supply is one that meets today’s needs without jeopardizing the ability to meet the needs of future generations.
  • Keep water affordable: An affordable water supply is one that provides water to the nation’s future citizenry at rates comparable to that of today.
  • Ensure adequate supplies: An adequate water supply is one that guarantees local and regional availability of water.

Desalination techniques:

1.      Distillation:

  • Multi-stage flash distillation (MSF)
  • Multiple-effect evaporator (MED|ME)
  • Vapor-compression evaporation (VC)

2.      Membrane processes:

  • Electrodialysis reversal (EDR)
  • Reverse osmosis (RO)

1.1. Multi-stage flash distillation

Multi-stage flash distillation (MSF) is a water desalination process that distills sea water by flashing a portion of the water into steam in multiple stages of what are essentially countercurrent heat exchangers. Multi-stage flash distillation plants produce [64%] percent of all desalinated water in the world, although a different type of desalinators, Reverse osmosis plants, are more numerous.

1.1.1. Principle:                                               

The plant has a series of spaces called stages, each containing a heat exchanger and a condensate collector. The sequence has a cold end and a hot end while intermediate stages have intermediate temperatures. The stages have different pressures corresponding to the boiling points of water at the stage temperatures. After the hot end there is a container called the brine heater. When the plant is operating in steady state, feed water at the cold inlet temperature flows, or is pumped, through the heat exchangers in the stages and warms up. When it reaches the brine heater it already has nearly the maximum temperature. In the heater, an amount of additional heat is added. After the heater, the water flows through valves back into the stages which have ever lower pressure and temperature. As it flows back through the stages the water is now called brine, to distinguish it from the inlet water. In each stage, as the brine enters, its temperature is above the boiling point at the pressure of the stage, and a small fraction of the brine water boils (“flashes”) to steam thereby reducing the temperature until equilibrium is reached. The resulting steam is a little hotter than the feed water in the heat exchanger. The steam cools and condenses against the heat exchanger tubes, thereby heating the feed water as described earlier.

Fig.1 The schematic representation for MSF technique

1.2. Multiple-effect distillation (MED)                                                                                              

Multiple-effect distillation is a distillation process often used for sea water desalination. It consists of multiple stages or “effects”. In each stage the feed water is heated by steam in tubes. Some of the water evaporates, and this steam flows into the tubes of the next stage, heating and evaporating more water. Each stage essentially reuses the energy from the previous stage.

The tubes can be submerged in the feed water, but more typically the feed water is sprayed on the top of a bank of horizontal tubes, and then drips from tube to tube until it is collected at the bottom of the stage.

1.2.1. Principle:                                                                                     

The plant can be seen as a sequence of closed spaces separated by tube walls, with a heat source in one end and a heat sink in the other end. Each space consists of two communicating subspaces, the exterior of the tubes of stage n and the interior of the tubes in stage n+1. Each space has a lower temperature and pressure than the previous space, and the tube walls have intermediate temperatures between the temperatures of the fluids on each side. The pressure in a space cannot be in equilibrium with the temperatures of the walls of both subspaces. It has an intermediate pressure. Then the pressure is too low or the temperature too high in the first subspace and the water evaporates. In the second subspace, the pressure is too high or the temperature too low and the vapor condenses. This carries evaporation energy from the warmer first subspace to the colder second subspace. At the second subspace the energy flows by conduction through the tube walls to the colder next space.

Fig.2 The schematic representation for MED technique

1.3. Vapor-compression desalination
                                                     

The VC operates mainly at a small scale, on small locations. The main mechanism is similar to MED except that it is based on compression of the vapor generated by evaporating water to a higher pressure, Which allows reuse of the vapor for supplying heat for the evaporating process.

Membrane desalination:

2.1. Electrodialysis reversal                                          

It is an  electro dialysis reversal water desalination membrane process that has been commercially used since the early 1960s. An electric current migrates dissolved salt ions, including  fluoridesnitrates and sulfates, through an electrodialysis stack consisting of alternating layers of cationic and anionic ion exchange membranes. Periodically, the direction of ion flow is reversed by reversing the polarity applied electric current.

2.2. Reverse osmosis                                                               

Reverse osmosis (RO) is a filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be “selective,” this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely.

Fig.3 The schematic representation for RO technique

Advantages and disadvantages of desalination techniques:

Desalination type Usage Advantages Disadvantages
Multi-stage flash distillation (MSF)

Desalination process that distills seawater by flashing a portion of the water into steam in multiple stages of what are essentially regenerative heat exchangers.

Accounts for 85% of all desalinated water; used since early 1950s MSF plants, especially large ones, produce a lot of waste heat and can therefore often be paired with cogeneration High operating costs when waste heat is not available for distillation. High rates of corrosion
Multiple-effect evaporator (MED|ME)

Using the heat from steam to evaporate water. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last.

Widely used, since 1845 High efficiency, while relatively inexpensive A large heating area is required
Vapor-compression evaporation (VC)

Evaporation method by which a blower, compressor or jet ejector is used to compress, and thus, increase the temperature of the vapor produced.

Mainly used for wastewater recovery Technique copes well with high salt content in water
Evaporation/condensation

Evaporation of seawater or brackish water and consecutive condensation of the generated humid air, mostly at ambient pressure.

Widely used Easiest method of distillation Time-consuming and inefficient in comparison to other techniques
Electrodialysis reversal (EDR)

Electrochemical separation process that removes ions and other charged species from water and other fluids.

Widely used, since early 1960s Long membrane lifetime and high efficiency (up to 94% water recovery, usually around 80%) High capital and operational costs
Reverse osmosis (RO)

Separation process that uses pressure to force a solvent through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side.

Widely used, first plant installed in Saudi Arabia in 1979 In water purification, effectively removes all types of contaminants to some extent Requires more pretreatment of the seawater and more maintenance than MSF plants
Nanofiltration (NF)

Nanofiltration membranes have a pore size in the order of nanometers and are increasingly being used for water desalination.

Emerging technology Very high efficiency High capital cost, unknown lifetime of membrane, no large-scale plant built yet
Membrane distillation (MD)

In membrane distillation, the driving force for desalination is the difference in vapor pressure of water across the membrane, rather than total pressure.

Widely used Low energy consumption, low fouling

Considerations in water desalination:

1. Cogeneration:

Cogeneration is the process of using excess heat from power production to accomplish another task. Theoretically any form of energy production could be used. However, the majority of desalination plants use either fossil fuels or nuclear power as their source of energy. Most plants is located in the Middle East or North Africa, due to their petroleum resources.

2. Economics:

A number of factors determine the capital and operating costs for desalination: capacity and type of facility, location, feed water, labor, energy, financing, and concentrate disposal

 Desalination stills now control pressure, temperature and brine concentrations to optimize the water extraction efficiency. In places far from the sea, like New Delhi, or in high places, like Mexico City, high transport costs would add to the high desalination costs. One needs to lift the water by 2,000 meters (6,600 ft), or transport it over more than 1,600 kilometers (990 mi) to get transport costs equal to the desalination costs. Thus, it may be more economical to transport fresh water from somewhere else than to desalinate it. Desalinated water is also expensive in places that are both somewhat far from the sea and somewhat high, such as Riyadh .Israel is now desalinating water at a cost of US$0.53 per cubic meter.[17] Singapore is desalinating water for US$0.49 per cubic meter

3. Environmental:

One of the main environmental considerations of ocean water desalination plants is the impact of the open ocean water intakes, especially when co-located with plants. These intakes are no longer viable without reducing mortality, by ninety percent, of the life in the ocean; the plankton, fish eggs. Other environmental concerns include air pollution and greenhouse gas emissions from the power plants. To limit the environmental impact of returning the brine to the ocean, it can be diluted with another stream of water entering the ocean. Discharges of brine into sea water have the potential to harm ecosystems, especially marine environments in regions with low turbidity and high evaporation that already have elevated salinity. Examples of such locations are the Persian Gulf, the Red Sea and The UAE, Qatar, Bahrain, Saudi Arabia, Kuwait and Iran have 120 desalination plants between them. These plants flush nearly 24 tons of chlorine, 65 tons of algae-harming antiscalants used to descale pipes, and around 300kg of copper into the Persian Gulf every day.

Conclusion:

Desalination process provides safe water and ensures the sustainability of the nation’s water supply. Egypt should do their best to use our sources such as solar energy and wind to use it as an inexpensive techniques for seawater desalination.

By

Amal  Sayed Moustafa Elsonbaty

Environmental Researcher