EXCESS SLUDGE PRODUCTION AND COSTS DUE TO PHOSPHORUS REMOVAL
Paul Etienne*, Laval Marie-Line, Spérandio Mathieu
Laboratory of Environmental Process Engineering. Department of Industrial Process Engineering, National Institute of Applied Sciences. 135 AV. de Rangueil 31077 Toulouse Cedex 4, France. E mail : paul@insa-tlse.fr
ABSTRACT
Based on a data collected from 35 French wastewater treatment plants and on published data, excess sludge production and chemical consumption associated with Phosphorus removal is estimated for the three following phosphorus removal processes : chemical precipitation, Enhanced Biological Phosphorus Removal and hybrid process. The influence of wastewater characteristics on excess sludge production are assessed. Chemical costs and costs associated with sludge disposal were calculated and results for the three Phosphorus removal processes are compared. The global cost for Phosphorus removal is then estimated.
Keywords : urban wastewater treatment, chemical precipitation, Biological phosphorus removal, sludge production, costs
INTRODUCTION
Phosphorus is considered one of the limiting nutrients in most freshwater lakes, reservoirs and rivers and so a low P concentration may control algae booms and eutrophication. Phosphorus inputs from point sources such as municipal sewage effluents are more amenable to control than from non-point sources. Therefore, regulations for phosphorus discharges in sensitive areas have been set by the EU Urban Wastewater Directive (97/271/EEC). In sewage wastewater, phosphorus comes mainly from human wastes and detergent (about 30% of total P in sewage in France [1]). During wastewater treatment, part of the soluble phosphate is transferred to a solid phase, generally entrapped into the organic sludges. This is achieved during normal biological degradation processes but can also be achieved by Enhanced Biological Nutrient Removal or by a physico-chemical process after chemical addition. It is evident that phosphorus removal increases the cost of wastewater treatment. This is due to investment costs, chemical costs and increased amounts of sludge to be disposed of.
Phosphorus removed from wastewater can be recycled together with sludges for land application, hence decreasing the phosphorus input from fertilisers. However contamination of sludge places this disposal route under more and more financial and social pressure. In addition, the agricultural market now demands a consistent and assured quality. The costs for land disposal of sludges is increasing and P removal may result in further additional costs for sludge disposal (reduction of land application rate and frequencies) [2]. Therefore, P recovery may be attractive if the sludge mass to be disposed of is significantly reduced [3].
This paper estimates the excess sludge production and the specific costs (�. Kg-1 Pinfluent) related to P removal in the urban wastewater treatment field. To reach this objective, we first explain the hypotheses made for calculations. Parameters such as wastewater characteristics, type of chemicals and chemical processes used for P-removal, the chemical dosage applied for precipitation, etc. are given based both on bibliographic data and on data from 35 French wastewater treatment plants where at least an 80% P removal is achieved (from a total of 77 plants which responded to our survey, 35 achieved this level of 80%). Based on the defined values for these parameters, the excess sludge production due to phosphorus removal is then calculated, considering different strategies for
P removal. Specific costs associated to P removal and also the total cost in France is finally estimated. In addition, the impact of phosphorus coming from detergents is discussed.
METHODS
Wastewater characteristics
Influent wastewater characteristics have a great importance on biological P-removal capacity. The contribution of P by population equivalent was first assessed. Values ranged between 1.7 [4,5,6] or 2 [7] to 2.7 g p.e.-1.d-1 P for wastewaters in England. Nowak [5] observed a decrease in this value when industrial wastewater is mixed with domestic. Our sample survey seems to confirm this tendency with lower values such as 1.3 to 1.5 encountered. A study made by Geoplus [1] gives values of 1.2 to 1.6 g capita-1.d-1 P (mean value 1.4 g Capita-1.d-1 P) for human wastes (urine + faeces), 0.3 g capita-1.d-1 P for food wastes, and 0.75 g Capita-1.d-1 P for detergents. This leads to a total P amount of around 2.5 g capita-1.d-1 P. A similar value is found from our sample survey. In this study we have therefore considered a value of 2.5 g p.e.-1.d-1 P as representative of P production in France.
For COD production we considered an average value of 135 g p.e.-1.d-1 COD characterised by a BOD/COD of 0.5. For 90 % COD removal, the COD removed will be 120 g p.e.-1.d-1 COD The mean ratios of COD/P and BOD/P are then around 50 and 25 respectively.
Type of dephosphatation process
The type of processes used and the nature of the chemical added are also required to assess costs associated with P-removal. Data from the survey are presented in this section.
Biological process176%Physico-chemical process47%
Figure 1 : percentage of the different treatment processes for P-removal. Results of the survey of French WWTP (total=47).
In the sample survey carried out in this study, the proportion of physico-chemical, EBPR and hybrid EBPR + physico-chemical processes is about 47% / 17% / 36% (figure 1). Simultaneous precipitation represents the majority of the physico-chemical processes used for P-removal.
Hybrid process11%Simultaneous precipitation89%Post-precipitation
Figure 2: proportion of the different physico-chemical treatment processes for P-removal. Results of the survey of French WWTP (total=39).
Chemical dosage
12@35Number of WWTPAl formsAl2(SO4)3FeClO4FeSO4FeCl3IronAluminium30252015105088%IronAluminium
Figure 3 : proportion of the different chemical agents used in chemical treatment processes for P-removal. Results of the survey of French WWTP (total=36).
Figure 4 : proportion of chemical agents used in chemical treatment processes for P-removal. Results of the survey of French WWTP (total=36).
Ninety three percent of the plants surveyed use Fe-based chemicals of in which FeCl3 (commercial 40% ferric chloride solution) represents 70%. This is certainly due to the simplicity of use of this liquid product. These observations lead us to base our calculations of excess sludge production and costs considering only ferric chloride. As the molar weight of Fe is higher than that of Al, the mineral excess sludge produced will be higher. The price of aluminium based product is much higher than the price of Fe-based product resulting in a significant increase in the specific cost (�.kg-1P).
244 250 Main price (E/t ) 200 150 100 50 0 103 76 105 FeCl3 FeSO4 Type of chemical precipitant FeClSO4 Aluminium form Figure 5 : mean price for the different chemicals used in the French WWTP concerned by the survey
Price variations are important and can be attributed to market fluctuations, transport costs and local parameters (such as distance from producer industries or equipment line…). For our calculations, we have considered the iron based chemicals to have a mean price of 100 �.t-1.
Anaerobic digestion is often used to treat or stab
