Reduced effluent concentrations

Processes for SS separation may fill three distinct functions in wastewater treatment, namely, pretreatment to protect subsequent processes and reduce their loadings to required levels, treatment to reduce effluent concentrations to required standards, and separation of solids to produce concentrated recycle streams required to maintain other processes. In the first two functions effluent quality is the prime consideration, but where the third function must be fulfilled along with one of the others, design attention must be given to conditions for both the separated solids (sludge) and the process effluent. 

Figure 1. Reduced effluent concentrations of Co(II)EDTA and Co(ffl)EDTA as a function of time and pore volume following the continuous injection of Co(II)EDTA through a packed bed of 0.1% MnOj coated silica (pH=6.5 and q=3.8 cm/h).

In most instances, however, and particularly in view of the tendency to move towards stricter effluent standards, some form of treatment is necessary in order to reduce effluent concentrations. The most common method for removal of inorganic heavy metals is chemical precipitation. However, for certain wastes and to meet particularly stringent standards more sophisticated treatments are necessary. These will be outlined in the following sections. 

The chief quantities based on tracer tests are summarized in Table 23-4. Effluent concentrations resulting from impulse and step inputs are designated Cg and C , respectively. The initial mean concentration resulting from an impulse of magnitude m into a vessel of volume is C = mfVr- The mean residence time is the ratio of the vessel volume to the volumetric flow rate, t = V fV or t = jo tCg dt/jo Cg dt. The reduced time is t = t/t. 

Fig. 2.5 illustrates the effect on the process response of increasing values of k. This shows that the response time of the system is decreased by increasing values of k and that the final effluent concentration leaving the tank is reduced in magnitude. Increasing k has, however, very little influence on the initial rate of response. 

The tray aeration method is a simple, low-maintenance method of aeration that does not use forced air.19 Water is allowed to cascade through several layers of slat trays to increase the exposed surface area for contact with air (Figure 18.9). Tray aeration is capable of removing 10 to 90% of some VOCs, with a usual efficiency of between 40 and 60%.53 This method cannot be used where low effluent concentrations are required, but could be a cost-effective method for reducing a certain amount of VOC concentration prior to activated carbon treatment. 

Some innovating treatment technologies may be introduced in the treatment of wastewater generated in the aluminum fluoride industry to make its effluent safer. The ion exchange process can be applied to the clarified solution to remove copper and chromium. At a very low concentration, these two pollutants can be removed by xanthate precipitation.24 A combination of lime and ferric sulfate coagulation will effectively reduce arsenic concentration in the wastewater. 

The HiPOx process achieved >99.9% reduction in MTBE concentration and easily met the treatment goal of reducing the concentration of MTBE to below 5 pg/L. However, significant concentrations of MTBE degradation intermediates and oxidation by-products were present in the final effluent. TBA was produced early during the chemical oxidation process. Its concentration was diminished by further oxidation, reaching below its regulatory limit of 12 pg/L in two of the three... 

The mechanism for sedimentation (clarification) is based on the density difference between SS and liquid. In addition, SS with larger particle sizes can settle down more easily. Rectangular tanks, circular tanks, combination flocculator-clarifiers, and stacked multilevel clarifiers can be used.14 Oliveira et al.15 reported that flocculation and sedimentation were conducted in the cassava meal industry and reduced the effluent concentration of organics from 14,000 to 2000 mg/L in the bench-scale reactor, with a hydraulic retention time (HRT) of 37 min. 

This set of equations is solved by ODE with t = 5. For comparison, the case with constant input temperature also is done. The steady state effluent concentration is 0.255 at constant temperature, but falls to 0.22 on the average with fluctuating temperature. The difference between these values of course can be reduced by reducing the temperature fluctuations which are high,... 

Clean water was flushed through the tank for several weeks in order to reduce contaminant concentrations. Water was then pumped from the tank to lower the water table to the 1 -m depth in preparation for excavation of the 14-40 SMZ. The 14-40 SMZ was manually excavated from the barrier cells starting with Cell 1 (farthest away from the tank wall). Two methods of emptying the cells were used. The first method involved using a small trash pump capable of pumping slurries. This method was somewhat effective and could be used successfully in laiger-scale applications where full-size pumps could be employed. The effluent from the trash pump was placed in a super sack, which acted as a filter to remove the SMZ from the water... 

EDTA chelation The EDTA chelation test evaluates the extent to which cationic metals (e.g., Al, Ba, Cd, Co, Cu) can be made less toxic or non-toxic by the addition of EDTA (Ethylenediaminetetraacetate). A cationic metal may be suspected as the cause of toxicity if both EDTA and sodium thiosulfate reduce toxicity. EDTA is typically added as a gradient of concentrations (based on its toxicity to the species of interest) to a single effluent concentration. 

Identification of the specific chemical(s) responsible for toxicity is not always necessary in order to develop sufficient control options for achieving and maintaining a consistently non-toxic effluent. For example, source investigations may identify an opportunity to recycle a heavily contaminated stream back to the process, for additional reagent recovery/savings, and reduced effluent toxicity (see case study example in Section 7). Treatment of individual concentrated sources prior to discharge could prevent the contamination of larger volumes that are more dilute and potentially more expensive to treat (Novak et al., 2002). 

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