Wastewater and sludge management and research in Oman: An overview

ABSTRACT

It is well recognized that management of wastewater and sludge is a critical environmental issue in many countries. Wastewater treatment and sludge production take place under different technical, economic, and social contexts, thus requiring different approaches and involving different solutions. In most cases, a regular and environmentally safe wastewater treatment and associated sludge management requires the development of realistic and enforceable regulations, as well as treatment systems appropriate to local circumstances. The main objective of this paper is to provide useful information about the current wastewater and sludge treatment, management, regulations, and research in Oman. Based on the review and discussion, the wastewater treatment and sludge management in Oman has been evolving over the years. Further, the land application of sewage sludge should encourage revision of existing standards, regulations, and policies for the management and beneficial use of sewage sludge in Oman.

Implications: Wastewater treatment and sludge management in Oman have been evolving over the years. Sludge utilization has been a challenge due to its association with human waste. Therefore, composting of sewage sludge is the best option in agriculture activities. Sludge and wastewater utilization can add up positively in the economic aspects of the country in terms of creating jobs and improving annual income rate. The number of research projects done on wastewater reuse and other ongoing ones related to the land application of sewage sludge should encourage revision of existing standards, regulations, and policies for the management and beneficial use of sewage sludge in Oman.

Introduction

The quantity of worldwide wastewater has been increasing rapidly in the last decades due to the rapid population growth and increased use of freshwater for various purposes. Wastewater if not properly treated can cause various harms, including threatening public health. Treated wastewater and sludge, which is a by-product from this treatment, can be resources under certain circumstances.

In the Sultanate of Oman, the majority of cities use septic and holding tanks to collect sewage water from residential areas. Many of these tanks quickly become overloaded due to inadequate construction and maintenance. Sewage from these tanks is transferred by municipal trucks to the closest sewage treatment plants (STP) or sometimes is discharged to nearby wadis (dry channel beds). The capital city Muscat is being fully connected to a piped sewer network, and the expectation is that a large amount of treated wastewater and sludge will be generated as a consequence. Proper planning and management of such resources will help to some extent to alleviate the acute water shortage problem of the country (Choudri et al., 2015). It was reported in 2010 that only 20% of Muscat’s individuals were connected to the sewage network, and with the implementation of sewage network project at the end of 2015, Haya Water has connected 86% of Muscat’s population to the sewer system (Zekri et al., 2014).

Overall, the aim of this paper is to provide an overview on the current state of wastewater treatment in the Sultanate considering infrastructure available, review of projects undertaken, and highlight of the costs involved in the treatment. A summary review of research and development works done for wastewater, graywater treatment, and reuse, as well as sludge studies done in Oman, is also provided.

Current status of wastewater treatment facilities in Oman

More than 402 sewage treatment plants (STPs) in the Sultanate are recorded in the database of the Ministry of Environment and Climate Affairs (MECA), half of which are in Muscat; some belong to the government sector and others to private owners such as hotels and industrial estates. In addition to the existing STPs in Muscat and Dhofar Governorates, the municipalities in theses governorates are executing projects to establish and operate integrated networks for collecting, transporting, and treating wastewater. In the early 1990s, the budget of the wastewater project for interior regions in Oman was recorded at 10 million Omani Rial (Mott Macdonald, 1991). That particular year’s project budget served nine towns: Khasab, Sur, Al-Buraimi, Al-Rustaq, Nizwa, Ibri, Ibra, Samail, and Saham. The amounts of treated effluent in the Sultanate in 2010 as recorded by the former Ministry of Regional Municipalities Environment and Water Resources are provided in Table 1 and Table 2.

Table 1. Capacity of treated wastewater plants in different regions in the Sultanate and their production rates (m³/day) in 2010.

Number	Wilayat	STP Capacity (m^3/day)	Reclaimed water (m^3/day)	Reclaimed water (m^3/year)
1	Nizwa	250	164	59,860
2	Nizwa (new)	450	219	79,935
3	Green Mountain	120	95	34,675
4	Bid Bid	250	98	35,770
5	Izki	180	131	47,815
6	Alhamra	120	38	13,870
7	Bahla	600	42	15,330
8	Manah	180	80	29,200
9	Adam	250	176	64,240
10	Samail (new)	2,700	962	351,130
11	Nizwa (new)	5,600	2,480	905,200
12	Ibri	1,800	1,412	515,380
13	Yanqal	250	185	67,525
14	Dhank	250	132	48,180
15	Hamra Droa	180	39	14,235
16	Mahdhah	2,500	197	71,905
17	Buraimi	2,500	980	357,700
18	Buraimi (New)	3,000	2,100	766,500
19	Khasab	600	258	94,170
20	Diba	120	107	39,055
21	Boukha	180	59	21,535
22	Sur	2,000	526	191,990
23	Jalan Bani Bu Hassan	600	224	81,760
24	Alkamil wa Alwafi	250	99	36,135
25	Jalan Bani Bu Ali	600	198	72,270
26	Masirah	1,800	1,392	508,080
27	Sur (New)	1,400	924	337,260
28	Ibra	4,500	770	281,050
29	Snaw	120	76	27,740
30	Mudhaibi	600	88	32,120
31	AlQabil	250	84	30,660
32	Bidiya	300	115	41,975
33	Aljaza	180	8	2,920
34	Shinas	600	211	77,015
35	Liwa	450	279	101,835
36	Saham	5,000	2,077	758,105
37	Suwaiq	120	56	20,440
38	Khabourah	120	90	32,850
39	Rustaq	7,200	2,520	919,800
40	Almusanah	450	193	70,445
41	Barka (new)	2,000	1,978	721,970
42	Barka	250	180	65,700
43	Haima	120	70	25,550
44	Mahout	120	55	20,075

Note. Source: Alkhamisi (2013).

STPs in the Sultanate produced about 37.446 Mm³ of treated effluent (TE) in 2000, with individual plant capacity from 8 m³/day to 15000 m³/day (Ministry of Regional Municipalities, Environment and Water Resources [MRMEWR], 2002), and produced about 97.8 Mm³ in 2010 (Al-Omairi et al., 2011), compared with only 24,000 m³/day in 1992 (Al-Zubairy, 1998). Wastewater privatization projects (Haya Water Company in Muscat and Oman Wastewater Services Company in Salalah) now provide centralized sewer systems and treatment to all of the areas of Muscat and Salalah Governorates. The main objectives of these companies are to set up a modern wastewater system to serve the citizens with new technology of operating, maintaining, and managing wastewater network.

The production capacity of the sewage treatment plant in the Salalah Governorate was recorded at 27,000 m³ per day recently (Al-Kathiri, 2014; Baawain et al., 2014b), with a plan of building two stages for increasing the current design capacity of the plant. This network served about 120,000 individuals in 2005 and around 200,000 people in 2010 (Al-Wahaibi, 2011). Further details of projected wastewater flow from domestic and industries in Oman are provided in Table 4, shown later.

Table 2. Treatment plant capacity in each catchment.

Number	Catchment	Existing STPs	Capacity (m^3/day)	New STPs	Capacity (m^3/day)
1	Al Seeb	Manuma	140
		Mabella	1,900
		Alkhoud	1,200
				Al Seeb	60,000
		Total for Al Seeb	3,240	Al Seeb Ultimate	80,000
2	Bausher	Al Ansab	14,000
		Baushar	420
		Shati Al Quaram	7,500
		Madinat Al Sultan Qaboos	2,500
				Al Ansab (phase 1) Al Ansab (phase 2)	55,000 25,000
		Total for Al Ansab	24,420		80,000
3	Darsait	Darsait	21,000
		Aynt	150
		Jibroo	150
				Darsait	50,000
		Total for Darsait	21,300		50,000
4	Al Amerat	Al Amerat	900
				Al Amerat	18,000
				Al Hajer	6,000
		Total for Al Amerat	900		24,000
5	Al Quriyat	Quriyat	300
				Quriyat	6,500
		Total for Al Quriyat	300		6,500

Note. Source: Al-Wahaibi (2011).

Table 3. Treated effluent tariffs and charges.

Description	Unit	Unit cost
Service fee for wastewater based on metered water consumption
Domestic tariff	R.O/m^3	0.154
Government and institution tariff	R.O/m^3	0.193
Commercial and Industrial properties tariff	R.O/m^3	0.231
Service fee for wastewater, where no water meter is available
Domestic tariff	R.O/month	5
Government, institution, commercial, and industrial tariff	Tariff to reflect actual usage and based on reasonable technical evaluations
Connection fee (monthly rental fee)
Domestic tariff	R.O/connection/month	2
Government and institution tariff	R.O/connection/month	5
Commercial and Industrial properties tariff	R.O/connection/month	5
Tankered wastewater	R.O/m^3	0.193
TE water sold to Muscat Municipality	R.O/m^3	0.193
Fee for installation of connections to new properties
Domestic tariff	R.O/connection	50
Government and institution tariff	R.O/connection	Actual cost
Commercial and Industrial properties tariff	R.O/connection	Actual cost

Note. Source: Al-Wahaibi (2011). 1 R.O = 2.58 USD.

Table 4. Wastewater flow projections for years 2013, 2015, 2020, and 2025 (m³/day).

2013	2015	2020	2025
90,865	165,750	324,134	364,282

Note. Source: Al-Muselhi (2014b).

Infrastructure of wastewater treatment in Oman

The treated wastewater reuse plays an important role in the management of water resources and environment, economic, and social aspects of a country. The wastewater projects in Oman are considered a significant element of all new developments. This section introduces the activities of Omani Wastewater Services Company, which is implementing a new sewage system in the Muscat Governorate. Oman has the capacity to accommodate the sludge generated each year. According to Alkhamisi (2013), agricultural land in Oman under cultivation is recorded at 72,299 ha. Fruits occupy the highest amount of land at 53%, then perennial forages (30%), followed by vegetables (11%) and finally field crops (6%). The highest total production in tons is from perennial forages (58%), then fruits (23%), vegetables (17%), and grain crops (2%).

Wastewater management in Muscat

Oman Wastewater Services Company or Haya Water (HW) Company is a joint company, owned by the government of the Sultanate of Oman. A ministerial decision number 31/2002 established it on December 17, 2002 to design and manage the wastewater collection and treatment system in the Governorate of Muscat. The Ministry of Finance owns the company and started commercial operations from January 1, 2006.

Before the Haya Water Company, sewage treatment plants (STPs) in the Muscat Governorate were owned and operated by the Muscat Municipality, but now all STPs are operated by HW under a concession agreement with the government of the Sultanate of Oman. The main objectives of HW are to plan a modern wastewater system to serve all the Wilayats of Muscat Governorate, operating, maintaining, and managing the wastewater network in Muscat, as well as building and operating the Bio-Solids Composting Project (composting of sludge). As mentioned earlier, all STPs that belong to this company generate currently (2011) an average volume of 84,144 m³/day of treated effluent, which will rise to 327,853 m³/day by 2025. Treated effluent will be used for road landscaping, golf courses, agriculture irrigation, industrial reuse, aquifer recharge, and potable water. It is estimated that Haya’s treated sewage will serve 290 km² of Muscat Governorate (Al-Muselhi, 2011).

Currently HW operates 12 STPs, which are located in old Al-Ansab, Darsit, Shattie Al Qurm, MadinatA Sultan Qaboos, Al Mabella, AlKhoud, Al-Amerat, Quraiyat, Bawsher, Al Manuma, Jibroo, and Aynat. All of these plants use tertiary wastewater treatment technology; in addition, some of these plants use membrane bioreactor technology (MBR) such as Madinat Al Sultan Qaboos and Shattie Al Qurm. Raw wastewater quality of one of the STPs (Al Ansab) is provided in Table 5, shown later. After the completion of HW projects in 2025, the above-mentioned plants will be connected to new five main STPs according to their catchments. Table 3 provides the capacities of existing (STPs) and five new ones.

Table 5. Raw wwastewater characteristics at Al Ansab STP, Muscat Governorate, Oman.

Parameter	Units	Minimum	Maximum
Biological oxygen demand (BOD5)	mg/L	350	400
Chemical oxygen demand (COD)	mg/L	600	900
Total suspended solids (TSS)	mg/L	350	500
Volatile suspended solids (VSS)	mg/L	280	400
VSS/TSS ratio	%	75	85
Total Kjeldahl nitrogen (TKN)	mg/L	50	70
Ammonia nitrogen (NH3-N)	mg/L	35	45
Total phosphorus (TP)	mg/L	9	15
Total alkalinity (as CaCO3)	mg/L	100	200
Oil and grease (O&G)	mg/L	—	200
pH	mg/L	6.0	8.0
Temperature	ºC	20	35

Note. Source: Al-Waheibi (2015).

The new Al Ansab STP is considered the biggest plant in Muscat. The construction of this plant went through two phases in order to expand its capacity for serving all major towns that belong to Muscat. The current production of treated effluent (TE) in phase I was recorded at 57,764 m³/day (Al-Muselhi, 2014a). Membrane bioreactor (MBR) technology is used in this plant; this technology is used to increase the energy efficiency of wastewater treatment plants in order to produce TE that complies with national standards for reuse in irrigation purposes and other usage such as recharging of aquifers.

According to the Arab Water Council (2011), sequence batch reactor (SBR) process and ultrafilter (UF) membrane systems are utilized at A’Seeb STP. SBR has been considered the most effective technology in removing nutrients and producing a high quality of sludge. All other sewage plants currently are at different construction phases, and will use either MBR or SBR systems as tertiary treatment fitted with ultrafiltration membrane (Al-Wahaibi, 2011).

Sludge composting initiative

Sludge originates mainly from the wastewater treatment process; the recycling of sludge generation depends on the process operation and efficiency of the plant, its type, cost, and its influences on the environment. There are many methods for utilizing sludge, such as gasification, converting it to fuel, and using it for manufacturing of bricks, but no legislations for these methods have been established yet in Oman. Based on the national Omani legislation of using municipal sludge in agricultural activities, composting will be the best option for reusing sewage sludge. Composting is a very useful and economic method as the compost contains a huge amount of organic materials, which saves power, reduces air and water pollution, improves soil in agriculture actions, and saves landfill space.

In 2010 Haya Water (HW) implemented a project to introduce an organic fertilizer compost called Kala compost, which is used for agricultural activities. It is a by-product of the water reuse treatment process. The composting plant has an area of 60,000 square meters and is located in Al-Amerat (Al-Maltaqa), which belongs to the Muscat Government. Composting is a biological process that depends mainly on the activity of naturally occurring microorganisms. The method of producing compost is to blend green waste, which consists of a mixture of grass clippings, tree trimmings, dry leaves, and cut shrubs, with dewatered sewage sludge using windrow technology. The green waste is generated by Muscat Municipality, while the biosolid (sludge) is generated by all HW reuse treatment plants.

Windrow technology is the most useful and cheapest method of treating waste material and the process consists of three stages: First, in the mixing stage, dewatered biosolids that make up 20% of the available amount of green waste are mixed. This mixture is then put into long piles with specific height and width where windrows are thus formed and stay there for 30 days. Second, in the turning stage, heat by bacterial activity during the aerobic state of organic material is released to above 55ºC for 15 days or longer to decompose plant seeds, plant pathogens, and human pathogens. Finally, during the curing stage the previous materials continue to decompose until the last decomposed raw materials are consumed by the remaining microorganisms. At this point, the compost becomes relatively stable and easy to handle after curing it for another 15 days. Samples of the final compost are tested to accomplish pathogen and heavy metal standards. Upon getting the laboratory results and ensuring that Class A of 1993 U.S. EPA guidelines has been achieved (ALSAFA Environmental and Technical Services, 2009), the composted materials become ready for packaging.

Cost recovery from wastewater in Oman

The developmental projects in Oman including all water resources and other infrastructure projects strive toward generating income through various means. Wastewater projects, for instance, could contribute to agriculture economy investments, creating job opportunities, and reducing reliance on public-sector funds. However, such projects have disadvantages of requiring large investments, ensuring adequate maintaining and operating cost of treatment plants, collection and transportation, and so on. Therefore, a proper tariff is necessary to achieve the cost recovery of such projects and provide financial stability.

A report on water reuse in the Arab world from principle to practice (Arab Water Council, 2011) states that HW Company in the Sultanate spent around US$4.3 billion to expand collection, transmission and treatment system of wastewater projects in Muscat Government, and about US$634 million was invested toward the construction of new wastewater treatment alone.

Treated effluent tariffs in Oman

According to the 30-year concession agreement between HW and the government of the Sultanate to implement and operate wastewater treatment projects in Muscat, the former adjusted the following tariffs and charges (Table 3) for the wastewater service fee based on metered water consumption and the places where no water metering is available. The bills, which are paid by the customers for water delivery services, also include the payment for this service. Al-Wahaibi (2011) reported that the proportion of households connected to the centralized sewer system in 2010 was around 23% and was expected to rise to 80% by 2018 and 93% by 2035.

Cost of municipal sludge utilization in Oman

The income from sludge utilization is a desirable aspect for every wastewater plant owner, as the price tag of converting the sludge into manufactured goods is quite high, and the marketability of its products that have human wastes in their ingredients is always considered as a challenge due to reluctance by consumers to use any product with known association with human body wastes. It is big challenge for government-owned entities to recover full costs from consumers in most countries. Federal irrigation projects in the United States are fortunate to cover 20% of their expenses from farmers (William and Liu, 2006).

Sludge-discarding service accounts for 40 to 60% of the construction charge of wastewater treatment plants (Veritas, 2000). All HW STPs and other companies are now sending their waste to the Kala compost plant in Al-Amerat region instead of dumping it in landfill sites. As a result, 100% of all sewage sludge that is generated by the company’s plants is now treated and converted to compost, which is sold commercially in the market (Times of Oman, 2012).

As reported by Times of Oman (2012), Kala compost was introduced to the market only in December 2010; the price of selling the compost is Omani Rial 15 for each ton. Based on ALSAFA (2009), the daily production capacity of the compost is 118 tons, which conforms to Class A of 1993 U.S. EPA legislation. The project has a significant positive economic benefit for the country in terms of job creation and enhancing resource recovery.

The government’s role in the management of treated wastewater and sludge production in Oman

Since the establishment of Ministry of Environment and Climate Affairs (MECA) under royal decree number 90/2007, the responsibilities of managing wastewater in all regions of the Sultanate have been transferred to this Ministry. However, the Ministry of Regional Municipality and Water Resources (MRMWR) shares the responsibility of managing wastewater in the rural region. MECA carries out several activities in its program, such as monitoring, inspection, sampling, analysis, and evaluation of all discharge to the environment in accordance with permits to discharge requirement (U.S. Agency for International Development [USAID], 2010).

Development of wastewater legislations

The government of the Sultanate realizes the importance of wastewater management, and since the 1980s, many policies have been issued to manage this source of water. The Omani legislations are issued either as laws in the form of royal decrees (RD) or as regulations in the form of ministerial decisions (MD). In 1984, Royal Decree number 45/84 created a Ministry for Environment and Water (MEW); thus, Oman was the first country in the Arabian Gulf to establish such a ministry. In 1986, special regulations for the discharge and reuse of wastewater and sludge were issued; these included prohibiting the discharge of wastewater and sludge into the environment in any form and under any condition without obtaining a permit from the ministry (MEW). In 1991, this ministry was merged with the Council for Conservation of the Environment and Prevention of Pollution into a new Ministry of Regional Municipalities, Environment and Water Resources (MRMEWR).

In 1993, MRMEWR amended the previous regulations for discharge of wastewater and sludge to be more integrated, clear, and in compliance with the latest technical and scientific developments; hence, the ministerial decision MD 145/93 dated June 13, 1993, “Regulations for Wastewater Re-use and Discharge,” was issued. It defines the uses of treated wastewater that comply with the applied standards and conditions for irrigating crops, grass, ornamental plants, and recharging aquifers, as well as the reuse of sludge under certain specifications (Table 5).

All of these regulations emphasize managing wastewater for two reasons:

• Protecting the environment and public health.

• Recycling sewage wastewater for agricultural usage and for beautification purposes.

In 2001, two laws were issued to ensure proper management of municipal wastewater and to protect the surroundings and public health. The first law is the law of “the Conservation of the Environment and Prevention of Pollution,” issued under Royal Decree RD 114/2001; article 20 of this law states that “it is prohibited to discharge hazardous waste and substance and other environmental pollutants in wadis, watercourses, groundwater recharge areas, rainwater, flood drainage system or aflaj and their channels discharge systems.” It is also prohibited to reuse or discharge treated wastewater without obtaining a permit from MRMEWR.

In compliance with the law just described, a second significant law, “The Law on Protection of Sources of Drinking Water from Pollution,” was issued under Royal Decree number 115/2001. This includes regulations for secure management of sewage wastewater and protection of groundwater against contamination (Table 6 and Table 7). In addition, this provides citizens with the best level of health and aims to protect land, soil, and water resources; however, MD145/93 became an appendix of this law.

Table 6. Wastewater maximum quality limits in Oman.

Parameter	Units	Standard A	Standard B
Biochemical oxygen demand (5 days at 20ºC)	mg/L	15	20
Chemical oxygen demand (COD)	mg/L	150	200
Suspended solids	mg/L	15	30
Total dissolved solids (TDS)	mg/L	1500	2000
Electrical conductivity (EC)	micro S/cm	2000	2700
Sodium absorption ratio (SAR)		10	10
pH		6-9	6-9
Aluminum	mg/L	5	5
Arsenic	mg/L	0.100	0.100
Barium	mg/L	1	2
Beryllium (Be)	mg/L	0.100	0.100
Boron (B)	mg/L	0.500	1
Cadmium (Cd)	mg/L	0.010	0.010
Chloride (Cl)	mg/L	650	650
Chromium (Cr)	mg/L	0.050	0.050
Cobalt (Co)	mg/L	0.050	0.050
Copper (Cu)	mg/L	0.500	1
Cyanide (Cn)	mg/L	0.050	0.100
Fluoride (F)	mg/L	1	2
Iron (Fe)	mg/L	1	5
Lead (Pb)	mg/L	0.100	0.200
Lithium (Li)	mg/L	0.070	0.070
Magnesium (Mg)	mg/L	150	150
Manganese (Mn)	mg/L	0.100	0.500
Mercury (Hg)	mg/L	0.001	0.001
Molybdenum (Mo)	mg/L	0.010	0.050
Nickel (Ni)	mg/L	0.100	0.100
Nitrogen: ammoniacal (N)	mg/L	5	10
Nitrogen: nitrate (NO3)	mg/L	50	50
Nitrogen: organic (Kjeldhal) (N)	mg/L	5	10
Oil and grease	mg/L	0.500	0.500
Phenols (total)	mg/L	0.001	0.002
Phosphorus (P)	mg/L	30	30
Selenium (Se)	mg/L	0.020	0.020
Silver (Ag)	mg/L	0.010	0.010
Sodium (Na)	mg/L	200	300
Sulfate (SO4)	mg/L	400	400
Sulphide (S)	mg/L	0.100	0.100
Vanadium (V)	mg/L	0.100	0.100
Zinc (Zn)	mg/L	5	5
Fecal coliform bacteria	MPN/100 ml	200	1000
Viable nematode ova	Number/L	<1	<1

Note. Source: MECA, MD 145/93.

Table 7. Reuse of sludge in agriculture: Conditions for application to land.

Metal	Maximum concentration(mg/kg dry solids)	Maximum applicationrate (kg/ha/year)	Maximum permitted concentrationin soil (mg/kg dry solids)
Cadmium	20	0.15	3
Chromium	1000	10	400
Copper	1000	10	150
Lead	1000	15	30
Mercury	10	0.1	1
Molybdenum	20	0.1	3
Nickel	300	3	75
Selenium	50	0.15	5
Zinc	3000	15	300
After spreading of sludge there must be a minimum period of 3 weeks before grazing or harvesting of forage crops.
Sludge use is prohibited: • On soils whilst fruit or vegetables crops, other than fruit trees, are growing or being harvested.• For 6 months preceding the harvesting of fruit or vegetables that grow in contact with the soil and that are normally eaten raw.• On soils with pH < 7.0.

Note. Source: MECA, MD 145/93.

The MRMEWR issued MD number 421/98. It promoted the establishment of further wastewater treatment services and encouraged the residents of small rural populations to set up sanitation units, from which wastewater could be discharged to specially designed tanks according to the required technical specifications listed in the regulations of MD number 421/98.

Wastewater and sludge research in Oman

A considerable amount of research has been done on wastewater in Oman by various researchers. On the other hand, very little information about sludge research in Oman is available in published literature. Wastewater research focused on growing crops, impacts on soil, aquifer recharge using treated wastewater, and other relevant issues (Alkhamisi et al., in press; Al-Busaidi and Ahmed, 2014; Abdelrahman et al., 2011; Mahad et al., 2014a, 2014b, 2015). Sludge study looked at its quality and likely use in crop production and remediation of contaminated sites (Al-Busaidi, Shaharoona, and Mushtaque, 2015; Padmavathiamma et al., 2014; Al-Busaidi, 2014b). According to the latest report published by Haya Water Company (2016), Table 8 provides information on quality of treated wastewater in Oman.

Table 8. Treated wastewater quality from the Al Ansab plant parameters.

Treated wastewater quality parameters	Units	Effluent value from Al Ansab plant	Class A (agricultural irrigation permissible limits)
BOD	mg/L	<3	15
TSS	mg/L	1	15
NH3 an N	mg/l	<0.16	5
Organic N as N	mg/L	—	5
NO3 as N	mg/L	16	50
Total P as P	mg/L	3	30
pH	—	7	6-9
Fats, oil, and greases	mg/L	—	0.5
Total dissolved solids (TDS)	mg/L	—	1500
Total alkalinity (as CaCO3)	mg/L	—	—
Fecal coliform bacteria	MPN/100 ml	10	200
Visible helminth ova	Number/L	0	<1
Turbidity	NTU	—	—

Note. Source: Haya Water Company (2016).

One study was conducted with the objective of maximizing the use of treated wastewater, supplemented by groundwater, by identifying short-season crops, and changing the area under cultivation of such crops. Field studies were conducted to assess yield components of wheat, cowpea, and maize crops grown in rotation with reclaimed water for irrigation. Results show that by using treated wastewater conjunctively with groundwater (assuming irrigation salinity of 1 dS/m and treated wastewater [TWW] availability of 38,267 m³/day) the cropping area can be increased from 695 ha to 2245 ha of wheat, from 313 to 782 ha (250% increase) of cowpeas, and from 346 to 754 ha (318% increase) of maize. Of the total irrigation requirement 24.24 Mm³, 57.6% was to be met with TWW and 42.4% was to be met with groundwater. Field studies confirmed that the TWW irrigation increased the yield parameters of wheat, cowpea, and maize crops without any adverse effect (Alkhamisi et al., 2013; Alkhamisi et al., 2011). In a second study, TWW was used to compare two methods of irrigation water application: drip and raised furrow bed. The objective of this study was to modify the furrow system to a furrow bed system and evaluate its water use efficiency in comparison to drip irrigation system. The tested crop was wheat, which is cultivated as a forage crop for livestock and grain production in Oman. Each plot had either a drip irrigation system or a furrow bed of 60 cm width. The plots were divided randomly using a complete block design with two treatments (water source: freshwater and TWW; irrigation method: furrow bed and drip) and three replications. Wheat was sown and all required parameters for soil and plants were measured. Plants were irrigated daily by drippers or 5 days per week by furrows based on crop evapotranspiration value. From soil salinity data, it was found that both methods added some salts to the root zone, with less salts found with the furrow bed method due to a heavy leaching process that occurred during irrigation. However, the general data didn’t show a significant difference (p > 0.05) in soil salinity between the two irrigation methods. Since TWW has some extra nutrients compared to freshwater, plant growth was better with TWW and almost all growth parameters were higher with TWW compared to freshwater. Generally, all measured data collected from both irrigation methods didn’t show any significant difference. Water productivity data gave better results with the furrow bed compared to the drip method. This indicates the higher efficiency of furrow bed compared to old method of furrow irrigation. However, drip irrigation could be better in reducing water evaporation whereas furrow bed is an easy method in getting good yield with lower cost and high productivity (Al-Busaidi and Ahmed, 2014; Al-Busaidi et al., 2014).

Another study was done to see the feasibility of managed aquifer recharge (MAR) using treated wastewater. Data show that TWW volumes will increase from 7.6 Mm³ in 2003 to 70.9 Mm³ in 2035. HYDRUS three-dimensional (3D) simulations show that areas with sandy loam soils are suited for infiltration ponds. Numerical simulations with MODFLOW (in combination with PEST and GWM) show that injection wells can be used for recharge without causing undue water ponding. Numerical simulations also show that in order to maximize the amount of water injected into aquifer, MAR was subjected to the following constraints: limit groundwater mound below 5 m, and maximum allowable injection rate is 1000 m³/day. Results show that 68 injection wells with a total injection rate of 62,205 m³/day was found to be a feasible option; there will be a discharge of maximum 7,500 m³/day toward the sea and the injection rate of each well ranges from 200 to 1000 m³/day. Preliminary financial analysis has shown that a cost of US$0.353/m³ to 0.550/m³ will be incurred for further reverse osmosis (RO) membrane treatment and injection (Ebrahim et al., 2015; Zekri et al., 2014).

Al-Busaidi, Shaharoona, Al-Yahyai, and Ahmed (2015) conducted research to evaluate the suitability of treated wastewater for irrigating date palms and monitoring the partitioning of some heavy metals (e.g., Cu, Cr, Cd, Pb, Mn, Fe, Zn, etc.) among soil, plants, and fruits. Results showed that the concentrations of heavy metals in both groundwater and treated wastewater were within the international standards. There were significant variations in heavy metal concentrations in soil at studied locations. In most of the cases, the concentrations of heavy metals were relatively higher in soils irrigated with treated wastewater compared to the soils irrigated with groundwater. Generally, the concentrations of heavy metals in date palm leaves were not significantly different in plants irrigated with treated wastewater or groundwater. However, there were significant differences in the concentrations of heavy metals in date fruits irrigated with different sources of water. The concentrations of some metals (Fe, Zn, and Ni) in date fruits were higher in wastewater-irrigated plants, whereas other metals (Cu, Cd, Pb, and B) were higher in groundwater-treated plants. In all cases the concentrations were within the permissible limits. Thus, the long-term effects of treated wastewater did not indicate any adverse effects of irrigation using groundwater and waste water on fruit mineral composition, including heavy metals.

Another study aimed to identify means/tools to optimize treated wastewater reuse in conjunction with other available water resources by taking into consideration their quantity and quality, in addition to the agronomic, environmental, and economic components. The study was done in an open field at Sultan Qaboos University, Muscat, Oman. Three types of crops (radish, okra, and eggplant) were grown and irrigated by four types of waters (A: 50% groundwater and 50% treated wastewater, B: 100% groundwater, C: 75% treated wastewater and 25% groundwater, and D: 100% treated wastewater). Soil physicochemical properties did not show significant differences with treated wastewater irrigation as compared to groundwater. On other hand, some chemical properties significantly increased (p < 0.05) when treated wastewater was applied, such as total carbon and some major elements (N, P, K). Crop physical analysis showed significant increases in plant productivity when plants were irrigated with treated wastewater, with insignificant changes in heavy metals between treatments, and no biological contamination in crop yield was recorded (Al-Busaidi and Ahmed, 2015).

The growth of biofuel plants was evaluated under treated wastewater irrigation. It was found that Jatropha plants irrigated with treated wastewater gave the best growth in term of plant height and green yield compared to groundwater (Al-Busaidi, 2014a). Treatment of graywater and reuse have been extensive studied by researchers in Oman (Ahmed et al., 2008; Jamrah et al., 2008; Prathapar et al., 2005; Prathapar et al., 2008; Ahmed et al., 2005; Ahmed et al., 2004; Prathapar et al., 2004; Ahmed et al., 2003; Ahmed et al., 2012). These studies convincingly showed that graywater in individual households can be utilized in Oman, considering technical, economic, and environmental aspects. Some studies were done also on using wastewater (oil production water) generated in the oil industry in Oman (Al-Haddabi et al., 2004).

For biosolids applications in Oman, Haya Water Company has developed its pioneering Kala Composting Plant to enable the efficient reuse of sewage biosolids and green waste, enabling their conversion to a compost product that can be used for agriculture, landscaping, and individual gardens. However, high application of sewage biosolids could result in heavy metals accumulation and many health problems. Therefore, sewage biosolids applied to agricultural land must be treated, be tested, and meet provincial quality standards. The objective of this study was to evaluate the effect of different fertilizers, especially Kala compost, on the quality of soil and crops. The study was conducted at Sultan Qaboos University, College of Agricultural & Marine Sciences, Agricultural Experiments Station, in an open field with six commercial crops (cucumber, tomato, cabbage, lettuce, carrot, and potato). Kala application improved soil physiochemical properties by holding much water, reducing soil bulk density, and adding mix nutrients compared to NPK fertilizer. Good plant growth was observed with higher plant production and better water productivity in Kala compared to NPK treatments. Generally, it can be concluded that Kala compost was a good medium for plant growth, supporting plants with many elements needed for high production. Chemical analysis did not show any problem of heavy metal accumulation in either soil or plant samples. Biologically, all crops grown in this study were free from any harmful bacteria that could affect human health. Using Kala compost as a fertilizer will support organic farming practices but farmers should evaluate its applicability with long-run applications (Al-Busaidi, 2014b; Al-Busaidi, Shaharoona, and Mushtaque, 2015).

Another aspect of sewage sludge research has been the improvement of soil properties and investigation of heavy metals concentrations in both sludge and amended soils. Although land application of sewage sludge has been proven to be an effective disposal method mainly because it is rich in organic and inorganic plant nutrients, trace metals in sewage sludge are of particular concern in regard to their effects on human and animal health. Bioavailability of trace metals depends to a large extent on soil properties such as soil pH, redox potential, clay content, iron and manganese oxides, and organic matter (Rieuwerts et al., 1998), and on length and rate of sludge application. Another concern is the environmental and health risks posed by organic chemicals present in sewage sludge (Harrison et al., 2006).

Soils in Oman are characterized by high pH and high contents of Ca and Mg carbonates (Ministry of Agriculture and Fisheries [MAF], 1990). Mobility and bioavailability of most metals are decreased in alkaline soils (Sherene, 2010). Metals can also precipitate with OH⁻ from soil solution and form metal hydroxides (Basta and Tabatabai, 1992). Carbonates are identified as very effective adsorbents in removing metals from soil solution (Madrid and Diaz-Barrientos, 1992; Ahmad et al., 2012).

Omani standards for wastewater reuse and discharge were adapted from Food and Agricultural Organization (FAO) guidelines for trace metals in irrigation water (MRMEWR, 1993). However, it is imperative that policies, standards, and regulations about land application of sewage sludge rely on research projects done under environmental conditions concomitant to Oman. Therefore, it is essential to identify the existing examples of case studies and projects of the uses and land application of sewage sludge in Oman. A major concern about trace metals is their solubility in soils, and hence their bioavailability. Therefore, projects on the speciation of trace or heavy metals in calcareous soils after the application of different rates of sewage sludge are important to determine their solubility and mobility. Al-Dughaishi (2009) and Al-Saadi (2016) have carried out research projects on the effect of sewage sludge application on heavy metal speciation, movement, and bioavailability in calcareous soils of Oman. As types of wastewater treatment plants and technologies for processing sewage sludge vary in Oman (Al-Saadi et al., 2012), it was important to characterize these sludges and make recommendations for their use accordingly (Baawain et al., 2014a, 2014b, 2015). Another concern is the biological contamination posed by sewage sludge due to the presence of fecal coliform and pathogens. The effect of environmental conditions in Oman on fecal coliform and antibiotic resistant bacteria viability rate in sewage sludge was studied by Al-Bahry et al. (2009, 2014). Environmental pollution and contamination of underground water in Oman by different types of sludges were investigated by Al-Musharafi et al. (2013a, 2014). Bioavailability, bioaccumulation, and plant uptake of heavy metals from sewage sludge were researched by Al-Musharafi et al. (2013b, 2013c) and Al-Saadi (2016).

Conclusion

Based on the preceding discussion, the wastewater treatment and sludge management in Oman have been evolving over the years. Sludge utilization has been a challenge due to its association with human waste. Therefore, composting of sewage sludge is the best option in agriculture activities. Sludge and wastewater utilization can add up positively in the economic aspects of the country in terms of creating jobs and improving annual income rate. Financial analysis for Haya Water showed that sludge composting is the best selection. It is the cheapest option to implement and operate and is an excellent method in terms of withstanding any big change in key factors that manage the process, especially since there are no such guidelines in Oman for sludge practices and spreading schemes to date, as established in other countries (Oman Wastewater Service Company [OWSC], 2005). Research projects done on wastewater reuse and other ongoing ones related to the land application of sewage sludge should encourage revision of existing standards, regulations, and policies for the management and beneficial use of sewage sludge in Oman.

Additional information

Notes on contributors

Suaad Jaffar Abdul Khaliq

Suaad Jaffar Abdul Khaliq is an environmental specialist, currently working with the Ministry of Environment and Climate Affairs, Muscat, Sultanate of Oman.

Mushtaque Ahmed

Mushtaque Ahmed is an Associate Professor, Department of Soils, Water and Agricultural Engineering, College of Agricultural and Marine Sciences, Sultan Qaboos University, Oman.

Malik Al-Wardy

Malik Al-Wardy is an Assistant Professor, Department of Soils, Water and Agricultural Engineering, College of Agricultural and Marine Sciences, Sultan Qaboos University, Oman.

Ahmed Al-Busaidi

Ahmed Al-Busaidi works as Associate Researcher, Department of Soils, Water and Agricultural Engineering, College of Agricultural and Marine Sciences, Sultan Qaboos University, Oman.

B.S. Choudri

B.S. Choudri is a Senior Research Scientist at the Center for Environmental Studies and Research (CESAR), Sultan Qaboos University, Oman.