nutrient removal from sewage
5.2 rEgulAtory BAckground
5.4.3 Energy requirements and cost of conventional Bnr processesBnr processes
Figure 5.5 shows the typical energy requirements of a typical medium size WWTPs serving a population equivalent (PE) of 400,000. The majority of the energy demand (55%) is due to aeration requirements for the biological processes.
The energy requirements due to pumping can vary considerably depending on the morphology and plant configuration. Given, the capital and operating expenses of the anaerobic digestion process, it is usually implemented in WWTPs having a PE higher than 40,000 in order to realize energetic benefits within a reasonable time horizon.
To evaluate the impact of the implementation of nitrogen removal on the plant efficiency, it is important to document the increase in the treatment cost that is incurred due to the adoption of nitrification/denitrification. The implementation of BNR increases the cost mainly due to the increased aeration requirements of nitrification, internal nitrate recirculation and potentially the addition of chemicals to provide the organic carbon source for denitrification and enhanced biological phosphorus removal (EPRI, 2002).
In several cases, particularly in North America, retrofitting of existing WWTPs is required to include BNR together with organic carbon removal. In other cases new BNR WWTPs plants are developed. The BNR cost for new plants is very different from that of retrofits. The cost of new BNR plants depends on the influent loads and on the required treated effluent quality. The retrofit costs are more difficult to present and discuss as they are more site specific and vary significantly for any given plant size. They depend on the same parameters as the new BNR plants as well as on the layout and design of the existing WWTP (US EPA, 2007). Tables 5.3 and 5.4 summarize the total capital cost for the retrofitting of different BNR plants in Maryland and Connecticut respectively and Table 5.5 shows the capital and operating and maintenance (O&M) costs for new BNR plants and for retrofits. Clearly, the retrofits result in much lower construction and O&M cost compared to new BNR plants. Furthermore, the ratio of capital BNR cost/design capacity is very wide ranging from 16 $/(m3/d) to 5234 $/(m3/d) showing that BNR retrofit cost varies greatly depending on the specific case study. Even in WWTPs with similar design capacity, the BNR retrofit cost can vary significantly.
In terms of phosphorus removal, studies show that the cost is relatively low if the goal is to achieve total phosphorus concentration in the treated effluent up to 1 mgP/L. However, the cost increases significantly when the treated effluent should have a phosphorus concentration lower than 0.5 mgP/L.
Pre-treatment, 1%
Pumping, 9% Primary Sedimentaon, 2%
Aeraon, 55%
Secondary sedimentaon, 2%
Sludge recirculaon, 3%
Thickening, 1%
Anaerobic digeson, 9%
Dewatering, 9% Other, 5% Disinfecon, 4%
Figure 5.5 Contribution of various processes to the energy requirements of a typical medium sized WWTP having a population equivalent of 400,000.
Integration of energy efficient processes 83
table 5.3 Costs for upgrading WWTPs with BNR for Maryland (US EPA, 2007).
Facilities with
Little Patuxent 68,137 A2/O 7,263,879 107
Marlay Taylor (Pine Hill Run)
17,034 Schreiber 4,986,641 293
Maryland City 9464 Schreiber 1,375,866 145
Maryland
Pocomoke City 5300 Biolac 3,924,240 740
Poolesville 2366 SBR 1,593,640 674
Princess Anne 4770 Activated sludge
4,311,742 904
Seneca 18,927 MLE 34,886,034 1843
Sod Run 45,425 MLE 21,999,198 484
Taneytown 2650 SBR 3,808,298 1437
Thurmont 3785 MLE 3,122,264 825
Western Branch 113,562 Methanol 47,132,782 415 Westminster 18,927 Activated
sludge
5,274,444 279
Aberdeen 10,599 MLE 3,177,679 300
(Continued)
table 5.3 Costs for upgrading WWTPswith BNRfor Maryland (US EPA, 2007)
Back River 681,374 MLE 138,305,987 203
Ballenger 7571 Modified
Cox Creek 56,781 MLE 11,466,657 202
Cumberland 56,781 MLE 12,929,990 228
Denton 1703 Biolac 4,203,767 2468
Dorsey run 7571 Methanol 3,967,307 524
Emmitsburg 2839 Overland 2,562,722 903
Hagers town 30,283 Johannesburg 11,190,344 370
Havre DeGràce 7154 MLE 7,596,882 1062
Hurlock 7571 Bardenpho 5,200,000 687
Joppatowne 3596 MLE 2,433,205 677
Integration of energy efficient processes 85 table 5.4 Costs for upgrading WWTPs with BNR for Connecticut (US EPA, 2007).
Bristol Phase 1 40,693 MLE 649,320 16
Derby 11,470 MLE 3,513,514 306
East Hampton 14,763 MLE 890,548 60
East Windsor 9,464 MLE 1,407,617 149
Fairfield Phase 2 34,069 4-stage Bardenpho
14,235,676 418
Greenwich 45,425 MLE 703,809 16
Ledyard 908 SBR 4,752,461 5234
Milford BB Phase 1 11,735 4-stage Bardenpho
1,407,617 120
New Canaan 5,678 MLE 1,570,463 277
New Haven Phase 1 151,416 MLE 11,134,336 74
New London 37,854 MLE 3,495,615 92
Newtown 3,528 MLE 1,436,601 407
Norwalk Phase 1 56,781 MLE 1,548,379 27
Norwalk Phase 2 56,781 MLE 7,042,287 124
Portland 3,785 MLE 1,266,843 335
Seymour 11,091 MLE 379,597 34
Stratford Phase 1 43,532 4-stage Bardenpho
5.5 InnovAtIvE BIoprocESSES In thE MAInStrEAM And SIdEStrEAM
Two innovative BNR processes are the nitritation/denitritation process and the completely autotrophic nitrogen removal process (Figure 5.6). Nitritation/
denitritation is the oxidation of ammonium to nitrite and its subsequent reduction to gaseous nitrogen. The adoption of nitritation/denitritation as opposed to conventional nitrification/denitrification has significant advantages, since it theoretically reduces the oxygen demand up to 25% and requires up to 40% less external carbon. Furthermore, it decreases sludge production by 30% and carbon dioxide emissions by 20% (Gustavsson, 2010). To accomplish effective accumulation of nitrite, while arresting the formation of nitrates, the growth of the ammonium oxidizing bacteria (AOB) should be promoted, while the growth of nitrite oxidizing bacteria should be inhibited (Malamis et al. 2014). The NOB can be inhibited by maintaining a high free ammonia (FA) concentration (FA > 1 mgNH3 ⋅ L−1) or high free nitrous acid (FNA > 0.02 mgHNO2-N L−1) concentration in the biological reactor (Anthonisen et al. 1976; Vadivelu et al. 2007; Gu et al. 2012). The AOB are also favoured over the NOB at low dissolved oxygen (DO) concentrations during aerobic conditions (DO = 0.4 − 1.0 mg ⋅ L−1)(Blackburne et al. 2008a) and high temperature (30–40oC) (Hellinga et al. 1998). Significant cost savings can further arise when fermented sewage sludge is supplied as carbon source to accomplish table 5.5 Average BNR Costs for new small scale WWTPs (US EPA, 2007).
System 15 m3/d 40 m3/d 100 m3/d 190 m3/d 380 m3/d MLE process
Construction (2006$) 348,771 415,585 563,912 803,108 1,167,914 O&M (2006$) 37,263 43,515 60,553 61,636 122,699 4-Stage bardenpho process
Construction (2006$) 448,992 491,753 634,736 889,966 1,293,524 O&M (2006$) 64,353 70,604 90,462 117,551 162,169 3-Stage process
Construction (2006$) 388,859 444,983 589,302 837,851 1,220,029 O&M (2006$) 44,005 51,360 69,133 93,403 142,066 SBR process
Construction (2006$) 448,992 509,125 644,090 931,391 1,290,852 O&M (2006$) 34,321 41,799 60,185 82,862 122,577 Intermittent aeration process
Construction (2006$) 306,009 499,771 780,391 1,150,542 1,371,029 O&M (2006$) 34,321 41,799 60,185 62,862 122,577
Integration of energy efficient processes 87 the denitrification process (Mayer et al. 2009; Ji & Chen, 2010; Longo et al. 2015).
Short chain carbon sources produced from fermentation can promote nitritation/
denitritation and denitrifying phosphorus removal via nitrite from the anaerobic supernatant (Frison et al. 2013a).
Figure 5.6 Nitritation/denitritation and nitritation/anammox processes.
The nitritation/denitritation process has been successfully implemented to treat the sludge reject water resulting from the dewatering of the anaerobically digested sewage sludge (Frison et al. 2013b). In this situation, the nitrite accumulation is easy to be established since the reject water is characterized by high ammonium concentration which also results in high FA (>2 mgNH3 ⋅ L–1) for the typical pH in which it is found. The nitritation/denitritation can also be employed in the wastewater treatment line (Yang et al. 2007; Blackburne et al. 2008b). In this case the free ammonia and the temperature are lower. Therefore, effective control of the process is required to achieve the via nitrite pathway. Previous work has shown that an elevated nitrogen loading rate combined with low dissolved oxygen during the aerobic phase can accomplish complete nitrite accumulation (Katsou et al. 2015).
According to the classification made by US EPA, the treatment of reject water through nitritation/denitritation is an innovative process with very few full scale applications worldwide (US EPA, 2013).
An alternative short-cut nitrogen removal process which is increasingly being adopted to treat nitrogenous effluents, including sludge reject water in WWTPs is the completely autotrophic nitrogen removal process or deammonification process (as it is also known) In this process ammonium is partially oxidized to nitrite (50%) and subsequently the anoxic ammonium oxidation (anammox) process is employed to remove nitrogen. In latter process the anammox bacteria convert the remaining ammonium and the produced nitrite into gaseous nitrogen under anoxic conditions (Figure 5.6). The biochemical reaction also results in the production of a small amount of nitrate. The deammonification process has been successfully implemented as a side-stream process for treating the sludge reject water produced from the dewatering of anaerobically digested sewage sludge.
The relatively high temperature and high ammonia concentrations typically found in these recycle flows facilitate this process. Very few applications of the deammonification process exist for the treatment of the main wastewater treatment line. In this process, AOB should be promoted and NOB inhibited to allow nitrite accumulation. At the same time, the need for selective retention of the anammox
bacteria is required. A full scale deammonification plant has been installed at the Strass WWTP in Austria where a side stream deammonification process can provide seed for bioaugmentation in the full-plant. The deammonification process has the advantages of no requirements for external carbon source (since the process is completely autotrophic), very low aeration requirements (57% reduction compared to conventional nitrification/denitrification) and very low excess sludge production. The deammonification process is considered as innovative for the treatment of the reject water with several full scale plants worldwide and emerging/research for the treatment of municipal wastewater (US EPA, 2013).
The disadvantages of the anammox process are that the anammox bacteria grow very slowly, and are very sensitive to environmental conditions. Nitrate, nitrite, dissolved oxygen and organic matter can inhibit anammox activity. Therefore, process stability can be a challenge particularly for full scale applications.
Furthermore, the deammonification process does not remove phosphorus; it thus needs to be coupled with a phosphorus removal or recovery process. The DEMON®, SHARON-ANAMMOX, ANAMMOX®, Paques, ANITA-Mox, DeAmmon, CANON, OLAND are all autotrophic nitrogen removal processes.
Depending on the developed process nitritation and anammox can take place in one reactor or in separate reactors. Table 5.6 summarizes the oxygen and COD requirements, the produced sludge and the treatment cost for conventional BNR, nitritation/denitritation and deammonification.
table 5.6 Comparison of the conventional BNR with the advanced BNR processes.
process oxygen
1Total cost includes both capital and O&M cost.
Enhanced biological removal via nitrite can also be integrated with nitritation/
denitritation in order to remove phosphorus biologically. Studies have documented the occurrence of denitrifying phosphorus accumulating organisms (DPAOs) that can utilize nitrate or nitrite as electron acceptors instead of oxygen (Carvalho et al. 2007). The denitrifying phosphorus uptake can reduce the requirements for
Integration of energy efficient processes 89 organic matter and sludge production through the simultaneous denitrification and phosphorus uptake. Ji and Chen (2010) investigated denitrifying phosphorus removal via nitrite in an SBR treating synthetic wastewater using sludge fermentation liquid as an external carbon source. The soluble phosphorus removal achieved was 97.6%. The main difficulty in applying denitrifying phosphorus removal via nitrite is that the DPAO activity is inhibited when these bacteria are exposed to significant nitrite concentrations (Saito et al. 2004).