ASSESSMENT OF DIFFERENT SOLID-LIQUID SEPARATION TECHNIQUES FOR LIVESTOCK SLURRY
Creators: name: COCOLO, GIORGIA
Publisher: name: Universit� degli Studi di Milano
Year: 2014

description: Livestock manure is a valuable resource for fertilizing and amendment, due to the high concentrations of plant nutrients. However, the increasing intensification of livestock farms combined to an improper manure management led to nutrient losses to air, water and soil. Among the manure treatment technologies that allow to reduce the environmental impact of livestock activities, solid-liquid separation is a widely applied technique that leads to a more appropriate nutrient application to fields and, thus, minimize nutrient losses and their related environmental problems. The evaluation of the different separating system and the optimization of this process is crucial in order to improve the performances of the treatment systems and the minimization of the environmental impact related to manure management. The present study aims to: (i) evaluate the solid-liquid separation treatment in a treatment plant under different operating conditions; (ii) investigate the optimization of the separation process through natural and synthetic chemicals.
The performances of different solid liquid-separation techniques were evaluated under different operative conditions. In particular, the separation efficiency of two screw press separators were compared to the separation efficiency of a decanting centrifuge, in order to evaluate the separation system that allow a better biological removal nitrogen. Both the screw press and the decanting centrifuge treated digested slurry; moreover, the decanting centrifuge was run with the liquid fraction produced by the screw press and with the addition of flocculants. In all the trials, the decanting centrifuge presented a higher separation efficiency, producing a liquid fraction with a lower dry matter concentration and enabling a correct biological nitrogen removal. The effect of manure pre-treatments on solid liquid separation performances was also evaluated. In particular, the effect of manure acidification on three different separation techniques was studied. Pig slurry (pH = 7) and acidified pig slurry (pH = 5.5) were treated through (i) screw press, (ii) decanting centrifuge, (iii) flocculation combined to a belt thickener drainage. Acidified manure was easier to drain, leading to a lower retention of solid particles in the liquid fraction. This resulted in a decrease of the separation efficiency for all the analyzed parameters. Furthermore, The acidification caused the concentration in the solid fraction of particle related species to decrease, of dissolved monovalent species not to be affected, and of divalent cations to be increased.
Chemical pre-treatments were investigated in order to improve the separation efficiency of mechanical separators for nutrient, especially nitrogen and phosphorus. In particular, the utilization of natural flocculants (such as chitosan) was compared to a synthetic flocculation treatment. A laboratory test was run treating the manure produced by pig livestock and anaerobic digestion plants were treated. In particular, 20 different manure samples were used: (i) 5 samples were collected from farrow-to-finish pigs livestock; (ii) 5 samples were collected from finishing pigs livestock; (iii) 10 samples were collected from co-digestion plants for energy production. The collected samples were flocculated through two different treatments: (i) a synthetic treatment (PAC+POLY), consisting in the addition of aluminum polychloride and a branched cationic polymer of medium molecular weight; (ii) a natural treatment (CHITOSAN). Different flocculant dosages were evaluated thrugh a jar test. The optimal dosage was assessed through: the floc size, the separation efficiencies of weight, total solids and total suspended solids. The optimal dosage of both the chitosan and the cationic polymer emulsions resulted highly dependent on the manure sample characteristics. Both the synthetic and the natural treatments allowed to improve the separation efficiencies, retaining a larger amount of solids and nutrients in the solid fractions. However, the chitosan addition lead to the formation of flocs more weak and with a non-well-defined structure compared to the synthetic treatment. For this reason, the separation efficiency of dry matter, volatile solids, nitrogen, phosphorus and potassium was lower for the chitosan optimal dosage compared to the synthetic treatment.
Physico-chemical separation was also assessed through a pilot-scale experiment. The slurry was composed by the pig and cattle manure produced by 10 livestock farms and was treated by a pilot-scale flotating system working with the addition of aluminum polychloride and a branched cationic polymer of medium molecular weight. Optimal dosages were evaluated by a jar test. The pilot-scale separator allowed to separate the 67% of the total weight of the input slurry and the 88% of the total solids. The pilot-scale flotating system allowed separating a large amount of solid fraction, which can be more easily managed and treated through different processes. However, the input slurry presented a large amount of coarse particles, which led to the formation of bigger flocs that tended to settle instead of floating on the liquid surface. For this reason, the input slurry should be pre-treated in order to remove the lager particles from the liquid manure. Furthermore, linear polymers with a lower molecular weight could be more suitable for the flotating systems, since they lead to the formation of smaller aggregates that can float on the liquid surface more easily. Finally, the set-up of the pilot-scale flotator should be modified, in order to enhance the transportation of the solid particles to the liquid surface. In this way the solid-liquid separation of livestock slurries through flotation could be better evaluated.
An empirical model was finally evaluated in order to predict solid-liquid separation efficiency and thus identify the more suitable separation technique according to farms? needs. To define and validate the model, the necessary data were firstly collected through published papers. This bibliographical research allowed us to collect publications about solid liquid separation published from 2000 to 2010. Model definition and validation was carried out on each of the groups using the two methods: random and cross-validation. The use of additives, the animal species and the separation technique were identified to be the parameters that most affected the separation techniques. Therefore, the database was divided in 12 groups according to these parameters. The graphical analysis allowed us to distinguish several technological and operational conditions that affect separation efficiency. However, more parameters could be taken into account with more data. Furthermore, for 7 of the 14 subgroups it was possible to define and validate the predictive models. These present RRMSEs lower than 50% thus can be implemented in a decision support tool, enabling the identification of the most effective treatment option. For the remaining separation technologies and operative conditions, most of which include the use of flocculants, more available data are necessary to define and validate empirical predictive models.
The results gathered through the experiments described in the previous sections allow us to better evaluate the different solid-liquid separation technologies and to define the most suitable systems for a manure treatment plant. In particular, the required characteristics of the resulting liquid and solid fractions may differ according to the whole treatment scheme and its primary scope. In this context, the available separation techniques present different separation efficiencies and, thus, lead to the production of liquid and solid fractions that have different concentrations of dry matter and nutrients. E.g. chemical-mechanical separation have a higher separation efficiency for dry matter and nutrients, compared to mechanical separators. Separation performances are also affected by operative conditions, such as manure type, pre-treatments and the flow rate of the input slurry. Manure characteristics are also affected by manure pre-treatments, which could modify physical, electrochemical and chemical properties of the input manure and, thus, influence the separation efficiency. Natural polymers allow to improve the separation efficiency for dry matter and nutrients. However, the optimization of physico-mechanical separation systems results higher with the utilization of synthetic additives, which lead to the formation of more structured aggregates; even though they could cause toxicity problems in soils or when the separated fractions are treated through biological processes (e.g. nitrification-denitrification or composting). Furthermore, the optimal dosage is highly dependent to the characteristics of the treated manure, therefore it is difficult to define guidelines that could give indication of the optimal dosage according to manure properties such as the dry matter content.During the combination of physical or mechanical separation with coagulation and/or flocculation pre-treatments, the selection of the optimal dosage and the more suitable type of chemical is strongly affected by manure characteristics. Therefore, the performances of a determined separation process may be negatively affected by the utilization of the wrong additive type. The identification of the more suitable separation process could be supported by a predictive model that could estimate the separation efficiency of a defined separator under different operative conditions. An empirical predictive model could give good indications for the separation systems more affected by the dry matter concentration of the input slurry (e.g. filtration or pressurized filtration), but the result less precise for other separation systems.