The city of Al-Diwaniyah is located in the south-mid part of Iraq; it is located on the Al-Diwaniyah River which is a brunch of Euphrates River as shown in Figure 1, with a population of around 400,000 inhabitants and several small industries. Although only about 30% of the city is served, however, municipal and industrial wastewaters of the served area are connected to a wastewater treatment plant through sewer. The basic wastewater treatment process in Al-Diwaniyah Sewage treatment plant is activated sludge processes. The plant capacity is 4DWF is the design capacity of the plant and amount of about (0.925 m³/s) enters the plant. Four screw pumps channels were constructed, only two screw pumps were installed, working on (Duty-standby) system, and each one has 3366 m³/hr. The project consists of two streamlines, each one includes screen with a maximum screen chamber of (2DWF) to protect the rest of the wastewater treatment plant units. These screens are cleaned mechanically. Then followed by the detritor which where grit are removed via the big change in the channel flow section which lead to decrease the flow velocity, the detritor contains a circular scraper to remove the settled grit. The return sludge coming from the secondary sedimentation tanks are mixed with the wastewater that leaving the detritor in the distribution tank in order to have a well mixing before entering the aeration tanks. An activated sludge process (ASP) represents the secondary treatment of the plant. Two aeration tanks (16,500 m³) were constructed, each one has six vertical aerators with (44 Kw each) which were organic matters are degraded. The effluent from the aeration tanks are received via a mixing chamber and then be distributed to the two circular sedimentation tanks, each one has a 36m diameter. Before the final effluent from the secondary sedimentation tanks is discharged to the river, it passes into the chlorination tank in order to be disinfected. The wasted sludge from the secondary sedimentation tanks is collected in a holding tank where the settled sludge is pumped to the thickener tank and drying beds while the supernatant is pumped back to the distribution chamber [10] . Figure 2 show the general layout of Al-Diwaniyah wastewater treatment plant. The plant was planned to make the final effluent of 60 mg/L suspended solids and 40 mg/L BOD₅.

For period from January 2011 to July 2012, samples were collected and tested directly at the laboratory of the wastewater treatment plant at Al-Diwaniyah sewerage Authority. Samples were taken from the inlet of the wastewater treatment plant (after the grit chamber unit) and the effluent samples were collected at the final stage (after disinfection stage). The methods were used for the determination of the concentration of the parameters were followed that of standards methods [1] .

Data were tabulated and analysed using simple descriptive statistics. Independent (unpaired) T-test for two means and analysis of variance for multiple means were used. P ≤ 0.05 were considered significant throughout data analysis.

Table 1 shows the concentrations of some of the influent wastewater parameters for about 16 month between January 2011 and July 2012. The compositions of typical municipal wastewater were used to determine strength of parameters were tabulated in Table 2 [11] . High concentrated wastewater represents cases with low water consumption and/or infiltration while diluted wastewater (low) represents high water consumption and/or infiltration. To characterize the quality of waste water averages, standard deviations as well as maximum and minimum values were calculated for the main parameters from the data.

According to the information in tables (1and 2), the raw wastewater showed a characteristic pH of 7.78 ± 0.11, it can be seen that the BOD₅ concentration for the influent wastewater was in the medium range (328.6 ± 61.3 mg/L) ranging from (158.8 - 367.7 mg/l). One of the most important parameters for wastewater characteristics is Biochemical Oxygen Demand (BOD), since in employ the type of the biological treatment. It is interested to notice that the concentration of BOD₅, COD were increased significantly during the years of this study compared with the previous study achieved [12] . This is meaning that the biological loading was increased due to either a difference in the type of exerted matter or due to the low consumption of the water used leads to high concentration of BOD. Consequently, it is important to examine carefully the concentration levels of influent and effluent BOD to establish potential water quality impacts in order to regulate these discharges. The COD concentration at Al-Diwaniyah wastewater treatment plant (STW) was determined in the medium range also (429 ± 109.4 mg/L). In addition, the total suspended solids for the influent sewage (228.7 ± 30.4 mg/L) with 288.0 mg/l and 190.8 mg/l for a maximum and minimum respectively and therefore it considered as a medium classification. For the sulphate, the average concentration for the influent was (604.7 ± 202.4 mg/L) ranging from (1063.5 - 452.3) mg/l and thus it can be classified as a high strength according to typical wastewater classification [11] . Moreover, Chloride was also categorised as a high strength due to the fact the average concentration is (362.5 ± 19.9 mg/l) with 329.8 mg/l and 407.0 mg/l for the minimum and maximum concentrations.

Figure 3 shows the influent concentrations of the BOD, COD, and TSS for the wastewater treatment plant. It can be seen that the TSS and BOD were very consistence with a constant trend for the influent wastewater, however, it also important to note that COD was higher and fluctuated especially in cold weather which might be due the fact of the low activity of microbes in cold weather.

In order to establish a constant relationship between deferent measures, regression analysis was used. Therefore, the variation of influent BOD with the influent TSS was determined, due to the fact that TSS test can be conducted much faster than BOD₅, which will take5 days. Once the correlation has been established, TSS measurement can be used to good advantage for treatment plant control and operation. Figure 4 displays the regression equation between the influent BOD₅ and influent TSS.

The concentrations of some parameters for effluent wastewater samples for the period between January 2011 and July 2012 are tabulated in Table 3. It can be noticed that the concentration of the most parameters were exceeded the maximum allowable limits that for disposing the treated wastewater to the river and water bodies. Figure 5 show the effluent of BOD and TSS comparing with the Iraqi standards for exposing treated wastewaters.

Typical values for the ratio of BOD₅/COD for untreated municipal wastewater are in the range from (0.3 to 0.8) and for the final effluent in the range from (0.1 to 0.3) [13] .

The value of BOD₅/COD ratio for untreated wastewater for the Al-Diwaniyah wastewater treatment plant is in the range from (0.4 - 0.7) with an average of 0.6 ± 0.1as shown in Figure 6, while the ratio of BOD₅ to COD for the final effluent is in the range of (0.3 - 1.2) with an average of (0.6 ± 0.2). If the BOD/COD ratio for untreated wastewater is 0.5 or greater, the waste is considered to be easily treatable by biological means. If the ratio is below about 0.3, either the waste may have some toxic components or acclimated microorganisms may be required in its stabilization [14] .

From Table 4, the summary of Al-Diwaniyah sewage treatment plant performance, the results shows that there are removals for some main parameters had happened. Although the results of T-Test show that there is a significant difference between the influent and effluent concentrations of PH, BOD₅, COD, TSS and PO₄, however, these percentages of removal were very low compared to the design criteria of the sewage treatment work. In addition, there was no significant difference between the influent and effluent of SO₄, Cl and oil and grease content. Therefore, it is clear that the wastewater treatment is not working properly and that due many different reasons which is either the quality of the return activated Sludge (RAS) or insufficient aeration that unable the microorganisms to biodegrade the organic matters. In addition, the secondary sedimentation tank may be suffered from insufficient time to make sludge settled. In fact, it is difficult to specify the exact problem because

there was measurement were taken from different location after each individual unit. Consequently, the sewage treatment work is in need of design and/or operational improvement.

One of the important elements to the growth of algae and other biological organisms is Phosphorus. However, the amount of phosphorus compounds present in wastewater discharge has to be controlled in order to avoid noxious algal blooms occurred in surface water [14] . Because a human excreta, mainly urine, domestic wastewateris a rich source ofchlorides. It does not present a major pollution threat. But, Chloride ion concentration is an important factor to be considered if treated effluent is used for irrigation [14] [15] .

In general, it can be say that, surface water bodies are under serious risk as a result of indiscriminate discharge of polluted effluents from the inefficient treatment and from industrial and/or agricultural, and/or domestic/ sewage activities.